All the answers have been given.
You said you successfully saw your two computers on your main Xbox login on your new PC, but you never confirmed for us that you are in fact using that login to start MSFS on PC1 (itās possible they are different). You can see who you are logged in as in the top right corner of the program after you get into it. You could also start a purchase process in the MSFS store and see what login it tries to purchase from and cancel before you actually buy the product to see who it thinks youāre logged in as.
If you are using the same login on both computers AND MSFS on the other computer, and that login matches your Microsoft Store and the Xbox App logins on your new PC , and you are logged in the same absolutely everywhere (Microsoft Store, Xbox App, and MSFS on PC1), then you have a problem with your gaming services as noted by @BoomUga (Can't get the MSFS I bought from Microsoft - #20), and youāre going to have to click on Support at the top of the page and then click on āSubmit a (help) Requestā at the top of the next page and get help from Microsoft. @BoomUga listed the process theyāre likely going to tell you you have to go through.
In the meantime, did you pay cash for MSFS? If you used a Credit card, canāt you look through your statements and figure out when you purchased it.
Unfortunately, and itās a major reason I avoid purchasing things through the Microsoft Store, you do not get a receipt from them of what you purchased. I have zero idea why or who thought it would be a good idea to do this, but the upshot is your credit card is going to be your only proof of purchase, and it wonāt list what you purchased.
Anyway, at this point, it sounds like the only people who can help you are Microsoft and youāre going to have to open a ticket.
BTW, I assume as well that you didnāt purchase MSFS through Steam? You never mentioned, it, but, the question needs to be asked. If so, then, well, thereās your answer why itās not showing up in the Microsoft Store. The Steam purchase process is completely different, and not shareable back to Microsoft Store.