If you want to translate a sentence by using dictionary definitions and say "well, it could be that", you need to stop and think for a second about exactly what "I have my clean reasons" means. The honest answer is it's not a viable translation. Very often you just have to go with the low-hanging fruit (the most obvious solution). Know that francophones also misunderstand one another by taking the wrong meaning of a word. The resulting confusion is a just reward.
There are lots of French (and English) words with multiple meanings. Duo tries to teach us a few of them, thankfully. But even this is incomplete. You can learn a lot from Duo, but you will learn more by also using other sources, and of course if possible, actual conversation with a francophone!
"Propre" is not used to mean "personal". It has a variety of uses that are quite different from each other. http://www.larousse.fr/dictionnaires/francais-anglais/propre/63683
I've had that happen more than once on the German tree, not yet encountered it here. You are right, it's less than helpful. I did notice that sometimes, when you click on the report flag, there is an option, "My answer should not have been accepted". I guess that's what that's about. Sigh.