Lost data on Instinct 2 Solar

Which kind of data exactly, and why did it get lost? Activities, as long as they were correctly logged are still on the watch in the folder //GARMIN/Activity, and can be imported to Garmin manually. Daily metrics like HR, Steps, Calories, etc., is being deleted upon syncing. So if your watch got accidentally associated to a different account than your main one, then the data is lost.

It definitely did not get associated with a different account. But it is the daily stats.

It can happen without you doing it intentionally. I recommend to Remove and Add a Device From the Garmin Connect App | Garmin Customer Support, then log a few minutes of steps and HR, and record a test activity, wait till the next morning, sync, and see whether it helped. If not, then have a look at the Troubleshooting guide for (some) data missing after syncing for more tips. However, the lost daily stats won't be recovered anyway. It only prevents that you continue losing more of it.