When I append a rigged model and try to go to the edit or pose mode, it immediately snaps back to object mode. Sometimes it goes to both modes normally and suddenly starts switching back to object mode later. It seems to mostly happen when I have more than one armature. I also use rigify for the character model in case it has something to do with it though it still happens when I append rig that doesn't use rigify.
This video showcases the problem. When I first append the character, everything works fine. But as soon as I append the swords, I can no longer go to edit mode in all three armatures and pose mode in the swords.