First checkout https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/463724 - you need this for Wikibase to work properly with MCR (might be merged by the time you read this ...)

Now upload a file, and add more than 1 caption

Adding the first, adding any subsequent one fails

The reason is - if you do a wbsetlabel call with id: <a mediainfo entity id> and without baserevid, the backend tries to create a new entity with the id you sent. This is fine for the first wbsetlabel call (because the entity needs to be created), but won't work for subsequent calls, as the entity already exists

The fix is - retrieve lastrevid from the response to each wbsetlabel call, and send that value as baserevid with the next call