Minä hymyilen, koska minä olen kalassa. -> This answer should be accepted?
Yes, but the repetition of the subject is a bit unusual. I would use just one subject at most per sentence, unless it's an exceptionally long sentence or there are several different subjects.
somewhere 'minä' is compulsory and somewhere 'minä' is considered incorrect. This beta version is so annoying
minä hymyilen should be correct, too.
In this example, the app doesn't accept minä at all.
Minä shouls also be accepted at the beggining like: minä hymyilen ...