This has been issue since somewhere Dragonflight and never figured out what was causing it but now I found it...

In Threat Plates settings:
General -> Blizzard Settings -> Display
These option do not matter, have effect or are not applied on reload.

Only way to fix this bug is to go to WoW's own options:
Esc -> Options -> Game -> Graphics -> "Use UI Scale"
- Either uncheck apply and re-apply or change it to 65% and back to 64%
After this the the Threat Plates settings:
General -> Blizzard Settings -> Nameplates
Need to be reapplied ie. if you had Enemy Widht/Height 180/25 you need to to set them again 180/25 to take effect.

Now the clickable area is correct again.

Would be nice to not be needing to do this everytime you load into instance, game or /reload.

Cheers!

Edit:
Seems like the same issue is on other nameplates too when you reload the Clickable Area breaks.
I don't think it is fixable since when you login or /reload it breaks.
Setting UI scale manually with /set command doesn't work, the fix needs to be done through settings with "Apply" and then do the set with Threat Plates that I wrote above.

Maybe you can find solution or not.