This is a widely reported problem where devices that are actually linked to the Microsoft account do not appear on the device list on the Microsoft account, it is a server side bug that Microsoft need to fix.
Even if the device does not appear on your account, the Bitlocker Recovery Key may still exist on your account on this link.
https://account.microsoft.com/devices/recoverykey
Be sure to check any account (Personal, Work or School) account that may have been linked to the PC for the Bitlocker Recovery Key.
Are you currently locked out of any device by a Bitlocker Recovery Key Blue screen.