I looked into it by adding some debug code into where we're fetching the album art provided over MPRIS by Spotify and it seems the URIs they provide are returning a 404 and thus it can't actually get the album artwork. Looking into implementing a (hopefully) temporary fix.
Regarding Chrome, I noticed this as well on newer builds of Chromium + Electron that some applications are using. Nothing we can actually do about it, since most of the time it actually works as intended, like in Firefox. Sorry.