Anytime you transcode, you are locking yourself into a 1920 x 1080 raster and whatever codec/data rate chosen. The FrameFlex vectors at that point become metadata should you ever go back to the camera original as in an offline to camera original online conform.
If you have a LUT and FrameFlex region of interest on a shot, and you transcode, you should end up with a .new clip that has that baked in to the clip. I will test again to make sure, but that is how it’s supposed to work in order to go back to legacy systems.
The only way to get FrameFlex vectors to work at high quality is when live linking to the 2K+ camera original. Also keep in mind that any other VFX that affects image size is always done to the 1920 x 1080 output of the FrameFlex pipeline.
Michael