also getting this issue - the adult content check on new posts is consistently unchecked, even after toggling the option a few times.
i'm using firefox on linux, but i think i've been seeing the same thing on windows as well (also firefox).
update: for desktop, it seems this is only happening when using the overlay "quick" post button that's in the bottom corner of the homepage. when using the main posting page, it correctly marks as adult by default.
now that we're using this modal in some new places, ive flagged this to the team and we'll get it fixed soon. thank you!
This has been fixed and will be live soon!