Hi,
This is a known issue with the Devices page in Microsoft account where devices may disappear after a clean installation. Some users suggested it could be related to a computer name conflict, while some suggested this happened after a BIOS update. Microsoft has acknowledged the problem and stated that a fix is being worked on. However, since it has persisted for several months, it does not appear to be a high priority. Unfortunately, there is currently no action we can take on our end, so we will have to wait for Microsoft to roll out the fix on this. In the meantime, you may check this thread and this page for additional details.
Disclaimer: This is a non-Microsoft site but the page is owned by a former Windows MVP. The information is reliable, but be careful with ads. Research any product before downloading.