@silverpill Figured out why threads from Mitra where Pleroma cannot fetch the parent get split into multiple threads even though to the origin, it's the same thread.

Mitra does not always include the "context" field in Objects.

GH W3C repo post with "context": https://mitra.social/objects/019c520b-782e-1492-6084-a90879a11ddc
Reply in the same thread without "context" where I cannot see the parent of your post: https://mitra.social/objects/019c522b-68d8-7041-917e-a7c30287cbc4

There's no way for Pleroma to infer the context when it is missing and the full reply chain isn't established, so it creates a new thread.

GH W3C repo post from my view: https://fluffytail.org/notice/B3FLUEBHd0iZyzWgLo
Same reply where I cannot see the parent: https://fluffytail.org/notice/B3FOb0jaJxpEATUqAa