I faced this today for doing paragraph_items to block_content migration, From Drupal 7 to Drupal 9. In my case, The block_content(of XYZ type) in D9 is translatable and paragraph_items in D7 were not translatable, But the nodes to which they are attached are translatable in D7.
I had to migrate all the revisions of the paragraphs[Not going into the details of work around paragraph's item-id/revision-id and node's nid/tnid to create revisions/translations of block_content].
When I ran the migration I faced a problem where the 'revisions' tab of the source translation showed more revisions than the actual count.
After looking into the tables, I sensed something was wrong with the 'revision_translation_affected' flag in tha DB table block_content_field_revision. And, Then I came across this issue and #6 by @firewaller. I used that solution by overriding the block_content entity's storage handler and it solved the problem and revisions tab started showing correct data.