Hi 8088mph, welcome.
Not sure, does it work when writing to a real floppy drive? You might want to ask at the big mess o'wires site, my guess would be Xenix is doing something odd, or maybe it's confusing the disk size 400k vs 800k.
are you able to do tar tvf from the disk device and get back what it wrote?
Off the top of my head, and I barely remember this, there's several different floppy devices in the /dev/ directory which turn on different options, I don't recall what they mean, but I think some of them will eject the floppy after a close, and possibly some are different sizes.