Hi Mike,
Hope you're doing well.
- Close the VM client down.
- Open the Hyper-V Manager settings.
- Under 'Server', find the 'Enhanced Session Mode Policy'.
- Uncheck 'Allow Enhanced Session mode', apply the changes, then re-check it and apply again. This should reset the settings.
- Start the VM but do not connect to it immediately. Wait for the VM to start.
- Right-click the VM in the console, and you should see the 'Edit Session Settings' option.
- Click this, and the original dialogue to select resolution should appear again.
Best Regards