Does anyone know why my comments have changed to this layout overnight? Literally overnight, so I haven’t altered anything myself.
It could have helped if you mentioned OS name, LO version and save format. You should also have attached a sample file.
I could recreate the behaviour with great pain. It seems to occur when you change indents with the ruler on the paragraph the comment is attached to. Apparently, part of the (manual?) formatting is forwarded to the comment, but with a bit of indeterminism.
So, without any file to check the hypothesis to your case, I can just suggest you select the anchoring paragraph and remove direct formatting.
Formatting with styles is much more reliable and avoids such mishaps.