Some players have reported issues with achievements not working properly, it's been tricky to test for since there doesn't seem to be a very clear pattern as to why it's only for some people.
I've updated Game Maker's steamworks extension to the most recent version in hopes that it'll work a little more smoothly and mitigate this issue (maybe even fix it entirely? boy wouldn't that be nice lol)
Thanks again for all your feedback and patience!