@sirhenrythe5th (…and others having issues with random custom splash screen videos)
After freshly upgrading to the latest retropi image on both my pi3b+ and my pi4, I ran into this issue again and had to add one more step.
After appending all the desired videos to the list (separate folders in the ‘opt/retropi/supplementary/splashscreen’ folder), disable the randomizer if it is already enabled, enable the randomizer, and select ‘Randomize /etc/splash screens.list’. Now when I reboot both the pi3 and pi4, a random splashscreen plays properly every time.
I did try this with the splash screens placed in /home/pi/retropie/splashscreens but could not get it to work reliably no matter what I did. This is likely due to me having retropie on an sdcard and roms on a usb drive… the usb is often not ready early enough in the boot process for the videos to work. No mods to autostart.sh etc worked either. If you are doing full usb boot, it will probably work keeping the videos in your home splashscreen folder (moving them to the opt supplementary folder is more work anyways).