I was looking at some of my achievements and one of them (Treasure Seeker) has a tier where I need to have completed five maps. It says I've completed six but hasn't awarded me that tier so it says I have 6/5 but the tier is still red.

I believe there was a problem with this achievement at one point, though I don't recall if it ever went in the direction of increasing your count.

Have you checked the actual Treasure Map page to see what your count is there? There's also the 'force update' link that you could try, but if the two match not sure it would do any good (and while I don't see how it could, I would hate for it to mess anything up).

oh that actually did work, thank you! The two pages matched (they both said I'd completed 6) but I guess forcing it to sync made it realize I had actually completed a tier maybe?

Yay, I'm glad. Good to know it works ok for the future too c: