You’re absolutely right. I believe this will only happen if you’re working in HD, and only with non-square pixels (HDV, DVCPRO-HD). And only if you’re scaling it to a size smaller than the actual width of your sequence. So, for example, with HDV, your sequence settings are 1440×1080. If your final output in Compressor is anything smaller than 1440×810 (which is a square pixel, 16:9 size), you’ll get this stretching. Furthermore, it’s not just text or Title 3D text. It will be any generator. Believe it or not, this has been a bug since at least Final Cut 6.0 and they still haven’t fixed it.
You have two solutions. You can move your footage into a square pixel sequence (the ProRes 422 (HQ) 1920×1080 60i 48kHz preset, for example) and send that to Compressor. Be sure to check everything first and make sure Final Cut hasn’t distorted anything in a weird way.
The only other solution (and probably easier) is to export a Quicktime movie and bring it into Compressor.
Finally, if this annoys you enough, you should send Apple feedback.
Even more effective, I think, you can register for a free Apple Developer Connection membership and use their bug-reporting tool. I have nothing to base this on, but I think they pay more attention to these official bug reports than they do to user feedback.
Good luck,
– David