bad performance on mysql8 with self-linking, reciprocal connection types

Since upgrading our database to mysql8, our website has suffered from enormous slowdowns relating to many-to-many connections between posts of the same type, with queries for connected posts taking minutes to complete (even for new posts)

Setting reciprocal = false massively improves the performance, at the expense of a more complicated process for creating and displaying the connections

Is there any obvious reason why the performance would have degraded so much after the upgrade? is there something we can do with the query or schema to improve the performance? There are around 4400 rows in the wp_p2p table, and 18000 in the wp_posts table (only ~3500 published)