Good morning, @LiamN ...

This seems a bit odd to me.  Have you checked with your school's Canvas administrator or anyone from their Online Learning / eLearning / Distance Education team to see if they've implemented some kind of custom code at the Canvas account level that would change this setting every day?  Maybe they could shed some more light on this for you than we could here in the Community...since we don't have access to your school's Canvas environment or any of your courses.  If you don't get any answers from your teams there, maybe try reaching out to Canvas Support to see what they have to say.

How do I get help with Canvas as an instructor?

Looking forward to hearing back from you on this!  Good luck!