I happened to notice something like this earlier today. I watched all my AI trains pass a 70mph sign, then pass a 35mph sign, it did change the limit in the hud for a few seconds but changed back. So I "chase viewed"(the #2 key in driver) while hanging off the rear most unit on a train, repositioned my point of view so as to get smacked in the face by every trackside object and found the front of the AI train passes a second speed sign before the back of the train passes sign the first one, then the AI defaults itself to the previous sign as it clears that one, totally disreguarding the newer speed limit. Could this be happening in your case? If its this is the case, just make sure the trains fully clear a speed sign before passing the next one.