I installed the 1.2.3 firmware and the google drive version of ASCOM v6.5.15 beta with the bug fixes and rebooted before testing. The first time I clicked the Advanced checkbox the Advanced dialog came up, the mount beeped once and then a modal error dialog popped up with "Error" "Set Meridian Flip Behavior Fail" with an Ok button. This only occurred on the very first time I opened the dialog.
In addition there's another issue that I see that was in the previous version as well. When connecting the mount in NINA (Telescope connect in NINA), NINA tries to synchronize site settings to the mount (Telescope). A NINA dialog pops up showing the mount/telescope's Latitude, Longitude and Elevation and NINAs Latitude, Longitude and Elevation. This dialog has a few buttons, when I press the sync from NINA to mount/telescope button I receive back an error in NINA "Unable to set mount latitude to xx.xxx and longitude to xx.xxx". I see in the log that the ASCOM mount log:
12:55:25.666 SiteElevation Set Do not support elevation
and suspect this is the cause of NINA's error. Is this return value ("Do not support elevation") specified by ASCOM? I'm wondering if it is not specified by ASCOM if the server should be changed to succeed all SiteElevation Set commands so that NINA doesn't throw an error. If it is a standard way for a mount to indicate it doesn't support the command, please let me know so I can open a NINA bug report to ask them to ignore failure to set elevation with the "Do not support elevation" return value.