The "resolve_image_location" replaces all icons with the new monologo icon.

eg:
pix_icon('qrcode', '', 'attendance')
will get overwritten with monologo icon.
 
"qrcode" is set here:
https://github.com/moodle/moodle/blob/master/lib/outputlib.php#L2112

but then if the plugin comes from a mod-  the candidates array is completely reset here, removing the correct icon name "qrcode"

https://github.com/moodle/moodle/blob/master/lib/outputlib.php#L2123
 
that like should sit within the " if ($image === 'icon')" statement.
 
it was in that location in earlier patches - not sure why it was shifted outside it?