But it does not have a lithium battery like the Labs Z-stick?
Again utilizing schnapps it might be possible to first remove the driver (kmod-usb-serial-cp210x) that ships with TOS4.x and then manually try with opkg installing the driver from TOS3.x and see whether it makes a difference. It still could be a kernel issue and not the driver though.
I am not sure how familiar with schnapps’s rollback/export/import features you are. I do a bit of troubleshooting/debugging and schnapps is extremely handy for that purpose. Of course it requires a bit of time, each rollback, export/import or even reinstalling the snapshot from usb.
But of course I would not want to lead you on in messing up your system - always at your own risk.
One more thought springs to mind - is the manufacturer of Labs Z-stick providing firmware updates, if there is such feature, and is the latest installed?