Changing Windows Key did not Work

[rpmtl22]

I have 5 Windows computers with 2 being in my home office. In November 2024 I built a new office computer (CompA - dual boot Win10/Win11). The computer it replaced is now my "second" computer (CompB). The one it replaced (CompC) is dismantled and the parts recycled.

My Microsoft account is not adding my newest computer (CompA) to the list of devices. While trying find out why I discovered a licensing error might be the cause. I have paid for the "10 device" ESU program and need this new computer to be included. Only 4 of my 5 computers are currently being recognized.

My licensing ERROR

I used the (non OEM) Win10 Pro license key previously used on CompC for the new system (A). But I MADE A MISTAKE and it seems I entered the Windows activation key from one of my other 3 PCs that are elsewhere in my home (oops).  Windows accepted it (8 months ago) and I only noticed yesterday (in Activation) that "Windows is activated with a digital license linked to your Microsoft account",  while at the bottom of the screen appears "The last product key (ID: .....) you entered can't be used on this copy of Windows (0x803FA67)".  But the system activated ... why??

I then chose to "Change product key" and entered the Win10pro Key from the discarded PC (which is unique and not used on any other system) and it was accepted. The "The last product key ... can't be used" message no longer appears. 

What concerns me is that when I run Belarc Advisor to report the current Win10 key in the Registry it reports the same key as what was there before and is a key used on one of my other three systems. (I re-ran Belarc on that computer to conform).

Does anyone have any explanation of what's going on here??? 

Should I use slmgr /upk (deactivate) and slmgr /cpky (remove the product key from the registry) to deactivate the system and reactivate using the appropriate key? I'm reluctant to do that as "if it ain't broke" ... yet I need my main computer to be included on the devices support by the ESU program. 

Thanks for any advice.

R