@sedi1111  USB-C Prioritization only comes into play when running a daisy chain, and it allows systems that only support an older DisplayPort standard to get enough bandwidth to run a dual display daisy chain by limiting USB data speeds to 2.0 rather than 3.x. But it wouldn’t kill USB data entirely, and in any case macOS still doesn’t support DisplayPort daisy chaining. If you’re connecting over USB-C and getting a video signal, then it’s not a cable issue since a cable that can carry video can also carry at least USB 3.1 Gen 1. So if you’re current on updates with both the display and the Mac, this might be one of those annoying interoperability issues that shouldn’t exist but do anyway. There’s nothing to configure on either side of this setup to get USB data flowing when you’re using a suitable cable, which you are, I can only suggest that you keep an eye out for new updates. On Apple’s side, I’m betting they’ll arrive since no huge architectural change like Apple just implemented ever goes off without any hitches at all. This may be a firmware glitch of some kind. And I rather hope that it is, because then it’s likely to get fixed. If it’s an issue that has to be fixed on the Dell side, unfortunately Dell has expressed no real interest in addressing issues with their displays that only affect Apple systems, of which there have been several. Each time someone posts about an issue like that, if there’s an official response at all, it basically just says, “The Dell Displays team doesn’t test against or officially support Apple systems. Talk to Apple.” And of course Apple says to talk to Dell. Meanwhile, their mutual customers caught in the middle suffer.

Same issue here with Dell P2421DC and Mac mini M1. I also use an P2418D as second monitor, connected to the HDMI port. I noticed that if I disconnect the USB C cable from the running machine and then reconnect the downstream ports do work. Looks like an initialization issue during boot.

We are not alone:

M1 mini external webcam and USB-C monitor

I can echo the fault with a Dell U3419W.  No USB passthrough via USB-C from a Mac Mini M1.  The display works fine.  Rather irritating as these are meant to be universal standards.

I just did a small test.

Mac mini M1 with 2 monitors connected (P2421DC (USB C) and P2419D (HDMI)). Downstream USB on the USB C monitor not recognised by the Mac (hardware overview shows nothing on the USB C port).
Power down the USB-C connected monitor
Power on the monitor again.
USB ports on the monitor are detected and working (I see an USB hub in hardware overview, only USB 2 speed. Maybe dependent on type of USB device inserted).

In a similar way, if I simply unplug the USB-C cable from the back of the Mac Mini, then plug it in again, it works fine.

What with this and the Mac outputting YPbPr instead of RGB (over any output) with no option to change it, I'm left to wonder what all the hype is about Apple.

Same issue here with the U2721DE and a Logitech Brio on the Mac Mini M1

I found this problem with a U2721DE monitor purchased by my parents.

First, you should pull up the User's Guide for your monitor. If you don't have it, use your monitor's service tag to look it up on the Dell Support site.

Once you have it, we changed three settings on the monitor:

1. In the "Personalize" section, we turned "USB-C Charging" to Off. This setting controls if the monitor continues to provide power over USB-C, even if the computer is off. We have a Mac Mini M1, which has its own power adapter, so it doesn't get power over USB-C. If you have a laptop, and it gets power over USB-C, you will want to leave this turned on.

2. In the "Display" section, we turned "MST" to On. It actually might not be required for this to work, though. The documentation says it is used to support connecting a second display to the monitor's DisplayPort Out port (the port that has an orange plug in it). We only have the one monitor, so this step might not be required.

3. Also in the "Display" Section, we turned "USB-C Prioritization" to High Data Speed.

Once we changed those settings, the USB-A ports on the monitor worked.

Three notes:

• If you have something like a phone, and you want it to charge while the computer is asleep, go to the "Personalize" section and change "USB-A Charging" to On.

• If you leave the "USB-C Charging" setting turned On, and you are using USB-C for video, you will not be able to change the "USB-C Prioritization" setting (the setting will be disabled). One workaround is to unplug the USB-C cable, switch to a different video source (like HDMI), and then change the setting. If you don't have another video source, look in your manual for instructions. In the U2721DE User's Guide, look for the section with the title "Setting USB-C Prioritization when USB-C Charging is set to On During Power Off".

• When you change the "USB-C Prioritization" setting, the monitor might pop up a warning message. In our case (for the U2721DE), the warning message said "If your PC is not DP 1.4, this option will result in reduced screen performance.". DP 1.4 came out in 2016, so I think the Mac Mini M1 uses at least DP 1.4. It also warns you that USB devices will be disconnected while the setting is changed, so make sure you don't have any devices (like USB drives) plugged into the USB-A ports.

That's it! With those changes, the USB-A ports started working on our Mac Mini M1.

I have excacly the same mistake with the Dell P2720DC.
Nice to see that I'm not alone.

Before I got the Mac mini M1, I use my MacBook 2017 with a very cheat dongle and it worked both over USB-C and HDMI.
Now that I've got a Mac mini M1, the HDMI connection produces fickering after a few minutes. But then the USB-A ports from the monitor worked fine. So if this HDMI issue doesn't occur with your Mac mini, you might try that.
I have found in other forums, that the HDMI-controler in the Mac mini M1 has an issue, hope that it will be fixed soon.

Thank you so much, it works