When a blocked user is merged into another account and deleted, the block stays in the ipblocks table with the original ipb_user value. This violates the foreign key (to user.user_id) constraint (which is not enforced when using the MySQL backend). This leads to inconsistent state - the block should be removed when the blocked user is deleted.
What version of the extension etc are you using?
Looking at https://github.com/wikimedia/mediawiki-extensions-UserMerge/blob/d5a1bb3a4242c455cd8134cd7f45170670a92448/includes/MergeUser.php there is support for moving blocks