The Prisoner boss fight possible bug

Despite hitting him with a ballista when prompted he full heals after getting him around 25% health.

I am melee and have spent the entire fight standing next to ballistas because I have read you have to be quick, but I have failed this fight 5 times because the ballista does not stop the regen.

If I am missing a fundamental aspect of this fight please let me know but I can make 15 more clips exactly like the one below where no matter what he heals full health ever time I am supposed to use the ballista.

It feels like I have 1 second to get the shot off (not click the prompt, I mean for the arrow to actually hit him) but it takes 2-3 seconds for the shot to fire.

I did not have an issue with this fight last league at all, I don't know what is going on.

Here is a clip

The first shot in this clip is the only time I have stopped his regen in 20 tries despite doing nothing differently. All other tries he gets hit with the ballista but goes back to 100% health

I have the same issue...extremally frustrating.