Certainly, that's what I thought at first. Restarting the game, or Windows doesn't help though.
I've had one instance when the game started running out of memory after I played for very long and crossed all of south underrail on foot from Core City to SGS, but then the game wasn't missing textures, it was just CTD-ing
My second guess is that it could be savegame bloat, but I thought I should check if it's a known problem.
Btw, greetings from Bulgaria