I opened a bug on this here:
I received an update to the weather-widget and am getting this error now when
I try to rebuild:

? tag-invalid           : <icon> not allowed in appdata 
? tag-invalid           : stock icon is not valid [weather-widget] 
Validation of files failed

My understanding was that the desktop entry spec file was used to generate the desktop and xml files.  Both are generated to include the icon tag - but now according to appstream-util this isn't allowed in the xml file any longer.  I checked in /usr/share/appdata and did a random check on org.kde.plasma.colorpicker.appdata.xml among others and they also have the <icon> tag and also now fail.

I also looked here:
and it appears that the <icon> tag is valid./chap-CollectionData.html#sect-AppStream-XML

So it appears that we have an error in either the process which is autogenerating the appdata file or the validation program has a bug.
I can certainly hack around this, but would prefer not.