I would imaging that if it was my browser it would be affecting all manner of sites I use, which it does not.
By other posts here it seems to cover a variety browsers. Personally I use Safari. Apple did some years ago forcibly block third-party cookies, but other than that nothing special. I do use Apple Private Relay, but turning that off made no difference.
So I'm firming an opinion that the problem lies in phpBB code, or configuration. I know nothing about either.