clarkwinkelmann Thank you Clark! I found the solution was:
/app/vendor/flamarkt/taxonomies/migrations/20210401_000200_create_discussion_term_table.php
$table->primary(['discussion_id', 'term_id']);
// to
$table->primary(['discussion_id', 'term_id'], 'custom_primary_index');
and then delete the table from the DB. The CSS issue in backoffice I was having was a conflict with fof/dark-mode