Hi, I'm John! I will help you with this.
On the Windows PC that is missing, could you please go to Settings > Accounts > Your info and tell me what it shows at the top? Does it show a Microsoft email address, or does it say something like Local account?
Also, when you check https://account.microsoft.com/devices please make sure you are signed in with the same email address that you used on the PC. Even a different alias or secondary email can make it look like the device is not there.
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Regards,
John J.D.