So I went for a ride on a LGV train (TGV 6112 Autumn Clear weather) with a custom livery (a blue sncf one) created in the livery manager. Now, I do have some more custom liveries for that train and actually I saw more then one different livery in the game in one session already (the default one, then one custom blue and a custom white one). While I was underway in direction to Avignon, an AI train came my way on the other track BUT: Only the leading cab and some passenger cabs where there, something like 5 or 6 cars, and the rest of the train formation was missing. Unfortunately I dont have a screenshot (because of the speed) but I try to get another one if possible. Btw it is nice that custom liveries are randomly applied to the TGVs.
I've had the same issue with the Autoracks on Sandpatch on a few scenarios, I just don't think it game can load in the custom liveries fast enough when passing by.