Given how close those plan estimates are to each other, I would say your query was very fragile under 14.6, and it was just a matter of luck of how the statistics were computed that you got the better plan on the older version. As opposed to some important coding changes that happened between versions. To verify that, Can you force each version to choose the other plan, for example by fiddling with enable_nestedloop on one and enable_mergjoin on the other?
