In MySQL, the behavior differs between the parent and child sides of the FK. It will auto-create the index on the child side if one is missing. But the parent (referenced) side must already have an appropriate index on the referenced columns.
Prior to MySQL 8.4, it was sufficient for the parent table to have any index that begins with the referenced columns. This has become stricter in 8.4 with default settings, to now require a UNIQUE index with the exact referenced columns.
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In MySQL, the behavior differs between the parent and child sides of the FK. It will auto-create the index on the child side if one is missing. But the parent (referenced) side must already have an appropriate index on the referenced columns.
Prior to MySQL 8.4, it was sufficient for the parent table to have any index that begins with the referenced columns. This has become stricter in 8.4 with default settings, to now require a UNIQUE index with the exact referenced columns.