"I came to Japan last month."
I got the same problem. I reported. 先月は should not be wrong.
先月は日本に来ました。-> 先月 is the topic (indicated by は), so you're highlighting that it was last month you came to Japan ("When did you come to Japan?" -> "I came to Japan last month.")
先月,日本に来ました。-> Here, the fact that you came to Japan is more highlighted, and the time you did so (last month) is only extra information. ("Where are you on holiday?" -> "I came to Japan last month.")
(Came up with examples on the fly, don't hate.)
Just so you guys know, the app accepts 来た in place of 来ました. It's the informal form. I did it by accident :p