MacOs 12.6.3
Hello.
<Short description of the problem.>
No search neither quick search do find the keyword applied from "Edit XMP" by the sidecar file itself. Both search and quick search find the keyword depending on the IPTC field which is created when " Edit XMP - Options - Mode = XMP Update or create IPTC" .
Personally, I choosed XMP sidecar files because are a standard, and don't write data on the photo (looking for the work of keywording being future proof and not dependent of a specific software)
Effect: An XMP sidecar file generated in Xnview depends on IPTC for searchability. Its no clear to me about this XMP sidecar standardization
To reproduce:
1. <Step 1> Using "Settings - Metadata - Sidecar - Update or create XMP Sidecar" Apply a keyword from " Edit XMP" with "Options - Mode = XMP Update or create IPTC" Perform search of the keyword ( XMP- all Fields) or Quick search
2. <Step 2> "Clear all fields" in "Edit IPTC" Perform search of the keyword ( XMP- all Fields) or Quick search
Actual behaviour (bug): XNview XMP sidecar files are not searchable by themselves

Expected behaviour: XMP sidecar files from other software (Darktable in this case) are searchable in XnView by themselve without any IPTC field being involved.
A related last thing: Once you have "Cleared all fields" in a photo using "Edit IPTC" , subsequent writings of keywords in " Edit XMP - Options - Mode = XMP Update or create IPTC" will fail to replicate the related IPTC field on that photo ( looks like clearing IPTC fields once, prevents the option XMP Update or create IPTC)