Ex-Meta employee here. I worked at reality labs, perhaps in other orgs the situation is different. At Meta we did "fix-it weeks", more or less every quarter. At the beginning I was thrilled: leadership that actually cares about fixing bugs! Then reality hit: it's the worst possible decision for code and software quality. Basically this turned into: you are allowed to land all the possible crap you want. Then you have…
> That’s not to say we don’t fix important bugs during regular work; we absolutely do. But fixits recognize that there should be a place for handling the “this is slightly annoying but never quite urgent enough” class of problems.
So in their case, fixit week is mostly about smaller bugs, quality of life improvements and developer experience.