Hi,
is it happening immediately after user creation?
Users created inside a group will be "dragged" out of the group and placed in the top level.
I've seen that from time to time as well.
Unfortunately i did not find any cause or similarity between those rare user accounts where this is happening, yet.
Until now, i just double check if the user is still in the group in which i created it, otherwise i'll do a manual move.
Regards
Marco