mak points out that:

Btw, you should upgrade AppStream to >= 0.10.6, looks like you are way
below that version ^^ (and for asgen, use the 0.6.2 release from
Debian, or use Git master, but don't use the released tarball, since
that one is missing an important patch - there will be a 0.6.3 release
soonish).

NB: due to this https://github.com/ximion/appstream/commit/1e43075ccaf55dd269f2406d758e332fd95cda39 we'd likely have broken icons, so this should be patched to enable both 64x64 and 128x128 icons being downloaded so discover can have best UX. I've sent a mail to mak and apol to figure out how exactly this should work (because right now it really doesn't)

IMPORTANT: make sure contrib/apt-conf/50appstream is patched to enable all icon downloads (64 and 128px) we need this for discover to have icons