U2422HE, MacBook Air Apple M2, intermittently going black when connected via USB-C upstream port

My monitor has recently started going black when I'm in the middle of working on my Apple laptop. I'm connecting to the U2422HE via the USB-C upstream port (6), and the power and peripherals continue to work even though the display doesn’t.

The message displayed on the monitor once it has gone black and says "Scanning for signal" then "No USB-C signal from your device" - the LED light remains on, then it goes into the pulsing state as if in Standby. 

What's weird, is that my laptop thinks it is still connected to the blank monitor screen, since the application window I was viewing on the monitor is still 'off screen'. One time this happened I had system preferences open with display settings visible and it showed the Dell U2422HE as the second screen (while the monitor itself was claiming no signal). I have to unplug the USB-C cable from my laptop to gather the windows onto the laptop display. It is as if the video is still being sent to the monitor, but the monitor display doesn't know it.

I'm losing the display a few times a day, almost hourly. Unplugging and replugging  the cable into my laptop doesn't fix the problem, but I am often able to get the display working again if I power the monitor off using the button, before reconnecting. Sometimes this doesn’t work - so then I resort to removing all the cables, powering the monitor off and holding the power button down for 8 seconds. It seems to reliably start working again after that.

This problem appears to have started after I installed the firmware update M3B409. I installed this while I was troubleshooting why my new webcam wasn't being recognised. (I now believe this was because I had plugged it into the wrong port (8) - I had missed ports 12 and 13 entirely. Now I have my webcam working because I found the right port.)

After the monitor has gone blank, I’ve unplugged the USB-C cable and plugged it into a different laptop - the display didn’t show up on the monitor for that one either (but I did hear the sound notifying me that power is getting to that laptop). I also tried plugging in an HDMI cable between the monitor and the same laptop after display went, and the HDMI signal wasn’t recognised either (I changed the input to HDMI).

My issue with all the standard troubleshooting steps is they assume the problem is permanent, and this is intermittent. But I’ve gone through them anyway: I’ve checked the correct input source is selected, I've performed the monitor self-test, I've run the built-in diagnostics. I've reset to factory settings. I've checked the pins on the cable aren't damaged. I’ve tested the USB-C cable works connecting to a different laptop (as above, it works when it works and it doesn’t work when the intermittent problem is occurring). I’ve tested the HDMI cable in both these states too, as mentioned in previous paragraph.

I wonder if there is a way to reinstall the old firmware to see if problem persists then, since I hadn’t encountered it before that update?

For reference:

I am connecting my Macbook Air Apple M2 chip to the U2422HE Monitor via the DisplayPort USB-C cable provided by Dell, plugging into USB-C upstream port labelled DP (no. 6 in the instruction manual). I use the cable to power my laptop as well as for the display. The monitor is also a hub for my peripherals (e.g. mouse, webcam).