I was doing my first playthrough to understand the game before speedrunning it, and the boss ended up getting stuck. While his back was against the top right corner, he did a dash left, but got stuck in the dash animation, and I was able to freely kill him. Is this a new thing or already known about? Sorry I wasn't recording that run, sure it'd be nice to see it.

I have seen this glitch multiple times before when playing the game. I've known about it for approximately a year prior of my post here, and it isn't fixed yet (as of 8/10/23). I don't know what causes it, though it might have to do with the dash animation getting stuck on the top portion of the room, maybe even the decorative columns placed on either side. For me it's the top left corner that he usually gets stuck in. And yes, it makes it very easy to defeat him.

Speaking of boss strats, when the boss dashes towards you, swing your sword at him repeatedly. His movement into you prevents a gap (due to knockback from sword) forming between you, enabling you to keep hitting him over and over very quickly for a short period of time. Also, picking up the potion from the shop is advised in order to make the fight easier.