This is hardly a game crippling bug by any means, just something I noticed and thought I should mention.
On my barracks (level 21) when I go to upgrade it it says "Max Level", as do all of my non-maxed buildings.
On my resource buildings (level 25) they all say "Requirements Not Met", rather than saying max level.
I've noticed it a few times before but never paid it much mind until right now.