Let say img.jpg is an image without any IPTC or XMP fields (I removed them with XnViewMP cleanup metadata tool to be sure).
Using the IPTC/XMP Editor of XnViewMP ('XMP only' mode), please to edit :
City = my city
Location = my location
State/province = my state
Country = my country
and then, please to run the following Exiftool commands (using a recent version such as 8.96) :
$ exiftool -args -g1 -a -XMP-photoshop:* img.jpg -XMP-photoshop:City=my city
-XMP-photoshop:Country=my country
-XMP-photoshop:State=my state
-XMP-photoshop:Location=my location
$ exiftool -XMP-photoshop:City= -XMP-photoshop:Country= -XMP-photoshop:State= -XMP-photoshop:Location=my location= Warning: Tag 'XMP-photoshop:Location' does not exist
Nothing to do.
As you can check here (Adobe site) and here (Exiftool site)XMP-photoshop:Location doesn't exist in the photoshop namespace !
XnViewMp should use XMP-iptcCore:Location instead of XMP-photoshop:Location.
NB. Exiftool is able to parse any XML code so it is can display 'XMP-photoshop:Location' but it doesn't accept to write unknown tags.
let says that "Editor" means "IPTC/XMP Editor of XnViewMP in XMP only mode, and Exiftool means the last production release of Exiftool
> XnViewMp should use XMP-iptcCore:Location instead of XMP-photoshop:Location
In fact XnViewMP writes to XMP-iptcCore:Location and XMP-iptcCore:Location fields.
Setting a value to 'Location' with the Editor => this value is written to XMP-iptcCore:Location (it is ok) but also to XMP-photoshop:Location (it is wrong) into the file.
At the opposite, setting a value to XMP-iptcCore:Location with Exiftool => this value is shown only into the 'Location' field of the Editor (it is ok). And it is not possible to set a value to 'XMP-photoshop:Location' with Exiftool since it raises "Warning: Tag 'XMP-photoshop:Location' does not exist".
As a result, it is impossible to delete this non-valid 'XMP-photoshop' field into the file, neither with the Editor neither with Exiftool directly. It is probably not very important but it is not clean that XnViewMP produces wrong metadata into the files.
I don't know if this bug is a XnviewMP bug or if it is an Exiftool bug (I checked its history but I did not find anything related).