I don’t have a lot of info here to identify the issue but from the description it looks like it may be a result of some kind of email tracking or a theme conflict in place…
What email provider are you using with your discourse site?
Do they offer some kind of email tracking service?
If so, is that service enabled?
It is possible that such a service might be blocked by an ad blocker or privacy focused browser which would explain this behavior, but in this case the account activation page itself wouldn’t load if the link is modified by the tracker, it shouldn’t actually prevent someone from clicking the activation button once the page has loaded.
Do you have any theme/theme component installed that might be conflicting with the page?
Do you have browser error logs from affected users?