I think in this day and age it’s not awkward at all to ask a member at any point if they’re a bot or not. I’d suggest an option to issue an automated a “challenge” for new accounts off the hop.

I’d also suggest removing any mention of weird behaviour or something untoward seen or suspected.

I’m thinking a message along these lines:

Hello,

This is an automated challenge message from the instance.

If this is an account run by a bot, do nothing and this account will be flagged as a reposter / bot.

If you are a person, please reply to this message.

This way at least the member can discuss whether they make their posts by hand or whatever, and the situation can be cleared up.