I've been in contact with the Optec folks about this. It seems this is related to the Optec focuser not implementing the Halt command and apparently when Halt is called it results in issues inside of the driver.
You can try using POTH and disabling the HALT functionality. This should block the Halt command from getting to the driver, thus shielding the driver implementation from this.
Then connect SGP to the POTH focuser.
This isn't likely something we're going to change in SGP. We're following the ASCOM recommendation about testing for Halt by sending the halt command and catching an error if it fails. Changing the behavior in SGP would affect other focusers that don't behave in this manner. Unfortunately this seems to put the Optec driver into a bad state. It was mentioned that they're going to be upgrading their driver somewhat soon but I'm not sure if they'll be changing this behavior or not.
Let me know if that works.