Thank you for your report. I'm sorry it's taken so long for someone to get back to you. The usage tag that says may be offensive is for the noun, not the adjective. Of course saying that someone is transgender is not offensive, but I believe that saying someone is a transgender may be considered offensive, particularly if someone is using it in a way intended to be insulting. If you believe that is not the case, then please get back in touch. It's important to us that terms like this reflect usage within the communities they are about, so if trans people don't think using transgender as a noun can ever be offensive, then we'd like to know that.