Okay, you have finally found the correct way to report an issue with OpenMPT. Not my personal email address, not my ModArchive profile and certainly not my SoundCloud profile. Just so this is absolutely clear, with every extra message you send to me through a private communication channel you make it less likely that I am interested in helping you, because your request is certainly not the most important in the world that it requires my immediate answer. Next time, directly post your question on the OpenMPT forums and report bugs on the issue tracker, and you might get a faster reply (in particular since I am not the only person who could give you this answer).

Now for the actual answer to your bug report: OpenMPT's behaviour is fully intentional, as FastTracker 2 does not validate the default tempo field when reading it, so if this value is greater than 255 in an XM file, it does not clamp it to 255. You can easily verify this by saving a song with a tempo of 512 and it will play just as fast in FastTracker 2 or XMPlay as it does in OpenMPT. Not every XM player supports this feature but that's not a reason not to support it. If you are concerned with ultimate compatibility, just don't use a tempo greater than 255.