I've had this issue. It seems their sync between the unit and app is poorly coded. Based on the one time I had this issue it appeared that the unit or app or both thought the sessions were sync'd but in my case there was a communication issue during the sync and that messed with it. I can only assume once they start a sync they are setting a simple flag that it's been synced and not actually verifying the data on both devices matches, let alone verifying it with a checksum as they should be doing to assure not only it's on both ends but the data integrity is good on the app end. I wish they would give the user the ability to clear the sync and re-download everything from the unit.
@TommyD11730 to get your data off: I have yet to test this, but per the docs you should be able to download the garmin fit files from the unit to pc/mac either directly or definitely via the garmin connect app using the usb cable. Then you can use one of the conversion tools to convert from fit to csv. It's a pita over direct csv export but at least you'll get your data off the device.
Additionally I've found that on my phone if I only have cell access (like at the range vs home), communication between the unit and phone app can be problematic and cause these kind of sync issues. Now while at the range I keep my phone app closed and phone bluetooth off, which is overkill but no bluetooth, no chance they will talk to each other.
I do all my shooting then back at home I sync the app and unit and export in one step vs per session using the three dots in the session menu (before you click on any session), click export, then click the date above the sessions which selects all sessions, then click export and choose destination. I use email because then on my pc I download the file and leave the e-mail as a backup copy. This gives one excel file with a tab for every session. I verify each session is actually there in the excel, then delete sessions from both unit and app so they are clean for next range session. I choose not to use their cloud storage option, not that it would help if you can't get the data off the unit.