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: FAILED: ? 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: https://www.freedesktop.org/software/appstream/docs
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.