Yeah, it's not behaving like a ban, it's behaving like... well... it's acting like the post is behaving normally, they just don't show up! I think they're running vBull, and that usually does a good job of telling you things like being banned.
But then with the legacy code they're probably dealing with, who knows. The less-than-reputable previous owners pulled a lot of stupid shit in their attempts to "monetize" things; like when they added the mining bot, quadrupled down on the off-topic adverts that made the site unusable without an adblock, etc, etc. The current owners have done a decent job of gutting out all that junk, but who knows what nasties and gremlins are lurking in that codebase.
It's part of why when I made this site I only placed one adsense ad... and honestly I'm tempted to rip that out as not being worth the time or effort. Oh noes, I use up resources on my hosting that's absurd overkill for my needs that I was paying for anyways, not that.
But then SMF is so much less resource intensive than other forum software. Never was quite sure why.