File close sometimes doesnt unlock file in windows

[spiras]

Load a .flv video;
close it with File / Close or Ctrl+w;
go to file manager and try to rename or delete the file: it's still locked by Avidemux!
close Avidemux; now the file gets unlocked.

This doesn't happen with other types of video.

Ver. 2.6.5. r8897 on Vista