Which is known to use an outdated version. So maybe whatever causes this has already been fixed for a year.
If you have a problem in a specific forum and it does not happen at try.discourse.org or other forums it’s often helpful to contact the admin of that forum first. They can check settings and customizations they have added.
By the way: posting an unlimited invite link in public can cause spam in the forum. If admins don’t change the setting that invited users get a trust bonus and start at trust level 1, spammers can use the link to bypass the tl0 limitations.