The automatic language tag should only depend on the community language settings and user language settings. Specifically it will build the intersection of both, and if the result only contains one item (excluding "undetermined"), that is used as the post language. The instance doesnt matter at all. And there havent been any changes in this area in a while, so there should be no difference between 0.19.x versions.
I thought maybe the community language isnt federated properly but its identical on all mentioned instances. Also @[email protected] mentioned that French is enabled in user settings. There is clearly a bug but I cant think what else might be causing it.
Anyway please report such bugs directly on the issue tracker, otherwise I might not find out about it at all.