I downloaded a reskin from DP Simulations about a year and a half ago to put into a scenario (which is now on steam workshop). When I most recently opened that same scenario in the editor, I noticed that not only had the AI trains using that reskin disappeared, but the reskinned train was also not visible in the list of rolling stock in scenario editor, even with the correct assets ticked in the assets list. Has there been a reset I am not aware of which has wiped some reskins out?
Steam can do a File Verification that checks the .ap file in each folder. It has been know to remove files outside the .ap file especially if that .ap file is not the size expected.