When using [Right-Click on message in message list] -> "Move Message To ..." -> [then any target folder] on a collapsed thread only moves the first message of this thread instead of the whole thread. My aggregation scheme is "Current Activity, Threaded".
I can confirm this behaviour: If I try moving a thread by drag & drop it works correctly and the whole thread is moved. But if I try it using right-click on the thread (which is not expanded) only the first message is moved.
This bug has never been confirmed for a KDE PIM version that is based on KDE Frameworks (5.x). Those versions differ significantly from the old 4.x series. Therefore, I plan to close it in around two or three months. In the meantime, it is set to WAITINGFORINFO to give reporters the oportunity to check if it is still valid. As soon as someone confirms it for a recent version (at least 5.1, ideally even more recent), I'll gladly reopen it. Please understand that we lack the manpower to triage bugs reported for versions almost two years beyond their end of life.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.1 aka 15.12, preferably more recent), please open a new one unless it already exists. Thank you for all your input.