hm... never had a problem.
Maybe your link is flagged because it is generic? Also the the algorithm may have been "trained" by dislikes on the platform to be sceptical about certain URLs and if it is the generic one you may fall beyond the threshold twitter has defined...
I also never just link to the instance itself but generally have a specific link to a profile or some such thing.
Care to try that as well?
@dirkprimbs @ainmosni Sure. A link to a specific toot on mastodon.social works. A link to the site itself is considered harmful.