Hi.
I also have these issues but do have a cable that’s working perfectly. I have about 18 docks and as I’m working my through them, the first 7 all have this issue of the HDMI losing synch. All of the 7 tested so far work perfectly with my iPad Pros, M1 and M2.
It’s also worth noting that the iPads also had these issues and on certain docks they would also send out a pillarboxed image, as well as the usual letterboxed image when mirroring and a lower resolution image with stage manager. However, after one of the iPadOS updates (I can’t remember which one) nearly all of the problematic docks worked perfectly.
Anyway, here’s a video showing the USB C to HDMI DP-Alt mode cable that works. Yes, this is very restrictive, especially if you use your iPhone as a gaming device and use Ethernet etc. (which is what I do). However, until such times as the docks work flawlessly, this is at least a solid option for video only. Plus you can also charge vide MagSafe, again, not ideal but it’s a workaround for now.
BTW. I’ve been testing these types of cable and docks for a few years now with the iPad Pros and totally understand the frustration when having to swap to numerous monitors and TVs, use different HDMI or DP cables and all the variations of setting up. What’s even worse for me personally is that I have the most awesome USB C dock with every option you’d ever need, that works perfectly with the iPad Pros but just keeps losing video synch with the iPhone 15.
I have a Pro Max, so can’t guarantee that anything I’ve said here will work with the 15 or 15 Plus, as they do have restrictions on their USB C bus and as far as I can, I can’t see how they could work at 4K/60 in DP-Alt mode. Although, I could be wrong as Apple may be allowing certain video protocols over USB C on the 15 and 15 Plus, even if they exceed the 480Mb/s for those phones’ USB C ports.
This video is more designed for noobs, just skip through to where I’m showing the cable working.
Cheers,
Dave.
https://youtu.be/fSjAUu5MtTk