I've had the same issue with shulkers. The fire damage wouldn't be so bad except while they're on fire it seems likes there's a cooldown before another arrow will damage them. So after one shot you have to wait another 5 seconds or so to kill them instead of just firing two arrows to begin with.
Sometimes it takes longer, man. Just don't give up and you'll get it after awhile. I've gone through hundreds of villagers in a breeder before hitting one with mending. Stay determined and you'll get there.
We need option to toggle it if it is intended. I feel like i wanted to crouch for a reason, and i should get to decide when I stop. I've already died a couple of times only due to this mechanic, and it doesn't exactly feel intuitive.
I can confirm this just happened with the latest update. I have multiple spawner grinders with lights to turn them on/off and now they all are just on all the time.