Lady Thorn boss fight has been bugged for almost 2 years now.

Rkindaleft

Essentially what happens is the circle you're meant to stand in never starts moving for some people or stops occasionally halfway through the phase. For the person/persons that the circle doesn't move for, you basically have to hug someone for dear life who can actually see the circle. This means doing the HM is much, much harder than it was before the bug originated because the bat damage from staying outside the circle stacks up extremely fast on HM compared to regular veteran. There doesn't seem to be any trigger for what causes the bug to happen, it just happens in some runs for some players and not for others, hoping the next attempt might be different.

1) Bug originally reported late 2021 by a number of players, maybe came around with Waking Flame or Deadlands DLCs.

2) June 2023, the boss fight is still bugged. This has been reported many times with a couple responses from Rich on streams saying that they will look into it, with nothing since.

ShawnF

I heard that if players never use the scythe throw mechanic (with adds or that one boss), they don't get the non-moving circle bug on the final boss. Haven't had a chance to test it myself but if you're pushing for an achievement it might be worth a shot.

That's not to say that the bug shouldn't be fixed... but it doesn't seem to be a priority.