Was not having this problem with Rex until the latest patch...

I seem to constantly keep having to go back to find Rex. He's not unconscious, or anything, I'm just walking around exploring and one minute he's right behind me, then for no apparent reason I realize he's no where to be seen and I find him sitting someplace I was at and have to go back and cycle through the "wait here", "follow me to get him to follow me again, only to go through it all over again down the road. It's happened a half a dozen times in the last half an hour. The last time it happened, I did a fast travel from Camp Mcleran to Vault 11. Every other time it has been while I'm just wandering....

I don't have Rex, but pre and post patch Ed-E likes to wander off for no reason at all around the gun runners area

I know this doesn't help, but I've had the issue you describe with Rex before the last patch was even a thought. It annoys the heck out of me to have to track him down have to ask him to follow me.... again. Anyways now that the game is patched.... again, I'm curious to see what he does now, though your post doesn't give much hope that this is fixed.

Oh well, at least the game will win one award..... BGOTY (buggiest game of the year).