If they are using a recent version of TidyPlates, there is an issue with any mod (like nUI) that uses a viewport. In TidyPlates, you have to go to the Options->Interface->AddOns->TidyPlates Hub and check the box for "Compatibility Mode".
The TidyPlates FAQ mentions that using a viewport tends to screw up nameplates in general; I only noticed the TidyPlates problem after 5.4, so it may be something Blizzard changed.
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