…well am just quoting here what I read on launchpad, see link above. And if « host » might be heard as Desktop Environment, it makes sense as a DE is run by $USER not root.
Same topic there → 'thunderbird.tmp' folder in '~/Download' directory?
I hope it could also solve this → GiMP and thumbnails for .xcf files - #18 by Coeur-Noir for any snap-app producing thumbnails.