Hi, #snac author here. I've just read your message and tested following a Threads account, and the process seems to complete correctly, the log messages report the correct dialog and the account appears in my being-followed list (I still haven't received any message from that account, but it's still early, I think). What may have happened in your case is that the Accept message from Threads to your Follow request was lost or not sent for whatever reason, so for snac it's a follow not (yet) confirmed.
Can you tell me what Threads account are you trying with? I'll investigate.