If I had to hazard a guess - Tesla has the incorrect purchase date assigned in their database for your VIN and has been running semi-frequent batch-processes between this and the supercharger database. When you complain, they fix it in the supercharger database, but not at the source, so it regularly reoccurs.
Also makes me wonder whether or not unscrupulous Tesla employees may have been adding friends and family cars to the "free supercharging" database, and this is an attempt to locate and remove those cars. Internal fraud is an issue many companies must deal with from time to time.
I've found throughout our ownership a small number of Tesla employees with the insight and connections to correct challenging to solve (non-standard issues) on the backend. With the sales staff laid off, your best bet is to find a customer-service manager that you can stay in touch with if the problem reoccurs again.
The suggestion to sell-back FUSC may be another worthwhile option for you. I've been considering it myself, as it seems unlikely that I will ever consume the entire amount given that the bulk of our charging is done at home. Even if I were to take ten cross-country trips, I still would not make it back -- I'm dreaming of making a single country trip, but haven't found the time!