If I understand this correctly what is happening is that the pier flip is attempted but the mount does not do it. It stays in the original pointing state. SGP reports this as an error and gives up.
SGP’s method of doing the pier flip at a defined hour angle will be vulnerable to this because the destination pointing state does not depend on the hour angle, it’s a function of the mount model. Even with a mount that’s expected to flip at an hour angle of zero there can be small differences amounting to a few minutes.
There are a couple of ways to get round this:
ASCOM provides a DestinationSideOfPier(double rightAscension, double declination) method. This reports the pointing state that a slew to the defined position at this time will use. As the mount tracks past the meridian the DestinationSideOfPier (DSOP) for the current position will initially report pierWest but when a slew would do the pier flip it will change to reporting pierEast.
SGP could monitor the DSOP and when it reports the flip can be done it does it. No need to rely on the user setting a good value for the hour angle, rely on the mount to say what it will do.
Not all mounts implement DSOP. For these an hour angle based slew is the only option, however if the flip fails - as shown by the pointing state not changing - SGP could try again at the next opportunity.