visual bug maybe...
Do you mean crouch flying? That probably got removed. This is a game with lots of issues open (at the time that I'm writing this) though, so it being a bug is more likely.
I'm thinking maybe Brigadier broke. That's the command parser library, so maybe that could have either overflowed, crashed, or just can't handle anything
Hasn't been tested on 1.14.3 and prior yet.
Happens with other mobs I think. The Pillager is the only mob that can have its' crossbow break as far as I know
Attach the crash log if you can find it, so the devs know what happens.
It occurred on Optifine when I was messing around (Mojang doesn't support Optifine as far as I know), then reloading fixed it. On vanilla it does the same thing. (When you save and quit you have to kill the Java proccess to exit)