Unfortunately it seems that when a feature has been present for more than a year or two then Garmin loses interest in bug fixes. I've had problems with "Race an Activity" ever since I owned a Fenix 5x. However, I stumbled on the following workaround. This also works for me on the F955, but others have reported otherwise.
Basically, the best way to race an activity is not to race an activity. Instead:
Start by enabling the virtual partner data screen in the activity settings. You'll be asked you to set a target pace, but this will be ignored when racing an activity.
When you want to race an activity, select the activity type, but don't select "Training"/"Race an Activity". Instead, select "Navigation"/"Activities" (to race an activity saved in history) or "Navigation"/"Courses" (to race a downloaded course). Then select "Go" or "Do Course". You can do this before or after you start recording the new activity.
You can then scroll to the virtual partner screen and it'll work the way it's supposed to. You'll also get navigation features, such as a course map and an elevation plot. You won't get these features if you select "Race an Activity".
