I've not had this issue with my bettas, but I do not own that style tank.
Sounds like he feels insecure with the lack of hiding spots after the trim back.
Photo of the tank?
Is there anything buffering the outflow of the filter? Or holes poked in the line to the outflow to reduce it?
He has hiding spots. The outflow has a sponge on it slowing the flow and he is fine with it, sometimes putting his nose right against the outflow.
The anacharis is already starting to eliminate his ability to paste himself against the filter, but today he has decided to go beserk and swim quickly all over the tank.
It is as if he is seeing his reflection in the glass, but his is really moving. He isn't flaring, per say, at the glass, but he does have his fins at full sail, just no gill flaring.
Water parameters are:
Ammonia = 0
Nitrite = 0
Nitrate = 5-10 (hard to tell with API kit)
Tap water that comes out at 1 ppm of Ammonia which resolves in about 8 hours. Prime to condition the water.
He gets a 50% water change per week to stay on top of nitrates which reach the 10-20 range. Just him and a nerite snail, which he doesn't like in the tank and will swim around and occasionally nip at.
He eats well as long as you don't try to feed him right after you wake him up in the morning. Then he is a little clumsy.
He's only been in the tank for two weeks, so some of this behavior could anomalous, but he was hiding from the light for a few days.
Today's wild behavior is new.