Hi there! What I would suggest is that you choose an A time and stick to it for a few days to see if that helps iron things out. I'm not sure how much his A time varies if he's had a short nap but babies do often yawn if they're bored. Do you think if you changed his activity, even to a quiet book, you could keep him up for a consistent 1.5hrs? Instead of picking him up when you know he's about to wake, could you place a firm hand on his back or tummy and shhhhh him?
There is a period of time where babies will only short nap, usually around this age, and there's not a whole lot you can do about it. Tweaking the routine and trying some new strategies will help you determine whether that's what is happening for you LO. Hugs for you, short naps are brutal! If you can attempt to cobble to together an EASY, that would be helpful.