Yes. Garmin doesn’t share [*edit: historical] data very well with 3rd party apps. It can only be requested in 3 month bins & every activity can only be sent once. If it’s already been sent from Garmin but gets removed from Xert, you would need to completely reset the GC > Xert Sync and request it again.
IMO, best to sync historical data from Strava. Much easier. I’m not sure how much (if any) additional benefit you’ll get from adding older data. You’d need to ensure all the data in the system is free from errors - no power spikes or wonky power readings, or artificial gaps in your training history (e.g. you were actually riding/training but not recording with a PM, so the data isn’t uploaded to Xert)