There is no "kactivities" product in bugs.kde.org that i can find, but it is said in the bug reporting guide in this forum to file activity bugs there.

So here goes. When restarting a plasma 5 sessoin, all windows are opened in the first/active activity, and not correctly assigned to their activities. If you know of a bug with the same issue, let me know. If you think I should file it and you are a kde dude, please tell me where.

https://bugs.kde.org/show_bug.cgi?id=341930
Bad bug