To reproduce, go to https://beta.musicbrainz.org/artist/7e84f845-ac16-41fe-9ff8-df12eb32af55/releases and select two releases and click the button to add them for merging.
Then open a new tab, go to https://beta.musicbrainz.org/artist/7e84f845-ac16-41fe-9ff8-df12eb32af55/recordings and select two recordings and click the button to add those for merging.
Go back to the first tab and attempt to submit the edit to merge the releases and you get:

Fehler:

Caught exception in MusicBrainz::Server::Controller::Release->merge "Can't locate object method "all_mediums" via package "MusicBrainz::Server::Entity::Recording" at lib/MusicBrainz/Server/Controller/Release.pm line 449, line 3."

Stack-Trace:
line 449 MusicBrainz::Server::Controller::Release
line 126 MusicBrainz::Server::Controller::Role::Merge
line 77 MusicBrainz::Server::Controller::Role::Merge
line 332 MusicBrainz::Server
line 306 MusicBrainz::Server
line 333 MusicBrainz::Server
line 347 MusicBrainz::Server
line 392 MusicBrainz::Server

Anfragedaten:
$VAR1 = {
'query_parameters' => {},
'body_parameters' =>

Unknown macro: { 'merge.medium_positions.map.0.id' => '1110772', 'merge.medium_positions.map.0.name' => '', 'merge.as_auto_editor' => '1', 'merge.medium_positions.map.1.release_id' => '327327', 'merge.medium_positions.map.0.position' => '1', 'merge.medium_positions.map.1.id' => '327327', 'merge.medium_positions.map.0.release_id' => '1100955', 'merge.merging.1' => '327327', 'merge.target' => '1100955', 'merge.medium_positions.map.1.name' => '', 'merge.medium_positions.map.1.position' => '1', 'merge.edit_note' => '', 'merge.merge_strategy' => '1', 'merge.merging.0' => '1100955' }

};

Host: pino

Oberflächensprache: de

URL: https://beta.musicbrainz.org/release/merge