The comments left by others give a good sense of the answer. It does differ depending on the meme, with larger fandoms are very strict about what hijacking is, so it's replying to a prompt adding your own ideas about what should be added, changed, eliminated or even just commenting with off topic jokes or general chat, and a mod will step in and redirect the conversation to a discussion post. The idea is, long conversation threads that can ramble away from the prompt or try to add to the prompt are thought to discourage fillers and basically just make the prompt a bit messy and off-topic when the OP may just want a fill and not a conversation.

As others here have already said, this is a small fandom and the conversations that have been happening under prompts are enjoyed by many. It would kill the meme a bit if I stepped in every time someone made a suggestion or began a discussion under a prompt. I think the key thing is how the OP feels about it, and if the OP feels that their prompt is being hijacked, they are encouraged to either speak up if they feel comfortable doing so ("I'd rather a potential filler stick close to the prompt") or else if they don't feel comfortable doing that, I urge them to PM me or write me in the mod post and I'll step in for them and ask the anons to relocate to the discussion post / post their own prompt with the added details they want.

Basically, if the OP feels it's prompt hijacking, then it is. If the OP is happy with the additions from other anons, then it's all cool.