As described in https://community.metabrainz.org/t/boom-boom-not-recognized-as-a-work-when-pasting-in-add-relationship-dialog-on-add-standalone-recording/690757, https://musicbrainz.org/recording/create doesn't recognize MBIDs containing uppercase letters when they appear in URLs, e.g. https://musicbrainz.org/work/4c4b77c7-21a1-32a9-A29b-241ff99cb397 (note the uppercase "A").

Per https://datatracker.ietf.org/doc/html/rfc4122#section-3 (describing UUIDs), "The hexadecimal values "a" through "f" are output as lower case characters and are case insensitive on input."