Comments - Poor performance with MariaDB (11) and utf8mb4_unicode_ci (vs utf8mb4_general_ci)
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So not from the latest uca1400 family of collations?
When comparing columns across tables, indexes can be used when the collation matches (as its an order). Are you using ALTER TABLE ... CONVERT TO CHARACTER SET ... COLLATE ....? ALTER TABLE ... DEFAULT COLLATE is only going to change added columns to the table?
Check ANALYZE FORMAT=JSON or optimizer trace for a few more details what's going on.