stlava,
I assume you are referring to those that have been imported/merged in the Admin Tab versus the import tab...
With that in mind... I have a few questions:
1) Are they still logged in after a refresh of the forum site?
2) Are they sync'd correctly? (i.e. not duplicates syncd to the same phpBB ID?)
If they look sync'd correctly and a refresh does not show them logged out I may need to peek around a bit to see what is happening. PM me info on the site, etc. if you want me to peek around... probably will want to see database tables and 1 user login which has the problem so I can check the sessions table as well as phpbb id's, and general behavior.
Thanks,
Greg