These are simple test files, so not useful for subtle sync issues. But the caption starts at the nonzero point I had in the project.
First, create a simple test sequence similar to your import. Set the caption at the same point; doesn't matter whether in sync with the actual audio. Just not the timecode. Export as mxf op1a with embed, and reimport. That may tell us whether it is a mismatch of some type between the exporting system and PR's import.