Yeah, I build a 4 display setup for my son with 2 hdmi splitters and 3 almost identical displays ( model code in one was slightly different even though had bought 3 same displays ) and one older one from different manufacturer with same specs.

Many desktop environments only saw 3 displays OOB. KDE was able to use 4, but it was unstable, especially with games and restart juggled the display order. Some trial, error and tweaking fixed most problems, but there was still some serious unstability due to different display models and spitters inability to fully reconcile differences. So, in the end it was a hardware problem.

He still used that setup for a while.

When my son settled on i3 desktop, solution was to buy a 4K TV and spit it to 4 displays with xrandr. Worked perfectly even with games (window borderless). He still set up a hotkey that made the 4 displays in to one, if some app/game didn't like the split.

I took those 4 displays because I had a multi-device setup and I needed a new display and was tired of using HDMI switch.