"Tata" and "mężczyzna" are clearly masculine because of their meaning, even if they sound feminine and have feminine declension.
Some other words that work like that but are a bit less logical are for example "turysta" (a tourist) and "artysta" (an artist). Their feminine versions are "turystka" and "artystka". There are not many such exceptions, don't worry.
For some time we rejected it because it's a different construction, then we started accepting such answers.
However, "Mój tata kupuje rower dla mnie" sounds kinda strange to me. This repetition of "mój"/"mnie". If anything, I'd say "Tata kupuje rower dla mnie"...
Also, emphasizing "dla mnie" could feel like "for me, not for you!"
In which case, I guess it was just an oversight, and might be corrected at some point.
As for general fora, on the website (I can't speak for the App) there is a link on the main front page which takes you to https://forum.duolingo.com/ where there are multiple discussions that you can join, or start your own.