I've messed around quite a lot with this and it's all a tad confusing - for something so apparently simple it's bizarre ! I now strongly suspect that in this case it is a buggy driver implementation. I don't know which piece has the dodgy implementation, whether it be Astroberry/Debian/INDI et al or a straight defect in this particular driver.
I've used FTDI, Prolific, and the bargain bucket jobs for MANY years. I've never had a problem like this.
I'm pretty sure it's not the prolific hardware per se. If it was, they'd be out of business
Looks like my options are buy or make something.
I have the bits to make something, just can't locate a verified pin-out diagram. Looks like FLO will be getting an order
Oh, thanks for taking the time to make the screen shot.