First: Thx working on the great browser falkon!! When switching from an unsecure PW Manger to an secure one, the pw data should be copied a securely erased from unsecure storage. As simple workaround could be: show a Dialog that reminds to use 'export', 'remove all', 'switch pwmgr backend', 'import'. If a secure method would be choosen as default. This bug can not happen to new falkon users.
I can confirm this bug. This can lead to important personal data loss. I vote +1 on the the data Import dialog. If this is not possible, at least a Warning should be issued that switching will cause data loss.