On this level I somehow managed to complete it, but it says "Level completed with warnings".

cases hxy with a ha
cases hyx with b hb
rw [ha] at hb
rw [← add_zero x] at hb
rw [add_comm] at hb
rw [add_comm (x 0)] at hb

Took me a few seconds but I found the underlined warning which says:

As far as I understand it that shouldn't really be an issue (I guess there's some reason for using sorry in that declaration), although it's weird that cases never gave me that warning before.

But also, I don't really understand how I can have solved the level... I was aiming for

0 + x = 0 + x + a + b
rw [add_comm, ...]
0 + x = a + b + 0 + x
apply add_right_cancel
0 = a + b + 0
rw [add_zero]
0 = a + b
(then I'm not sure but I guess it's possible to prove a = b = 0 from here)

But I didn't actually do any of that - Lean never did that many steps for me automatically before..

The thing that is definitely a bug though, is that it says Level complete but Next is still greyed out.