OK cool thank you for your assistance and reply, @RBoy… I’m still trying to understand the different settings here, so please bear with me. In the First conversion task I also see this same error code by MP4Box but it happens to embed the subtitles as expected in this task.
I was also under the impression that Handbrake is used to embed subtitles as well, so not 100% certain that MP4Box is the culprit here (or at least do not understand it as of yet). A lot of moving pieces here for sure. 