libebur128 is not available and therefore Cantata can not scan for replaygain.
Can actually confirm this. Cantata can neither detect nor scan (hangs at 0%) for RG tags. And I know there are in the files cause I scanned every album when I was still using Arch. And right now I just doublechecked it on windows via foobar. The tags are there it's just cantata for some reason or another can't see it.
Well, not exactly. According to cdrummond at github it should use either a system one libebur128 or the bundled one. Now, since Solus does not have it packaged it should default to the bundled one. Problem is, it's not using it. So maybe it's packaged/configured wrongly?
It wasn't difficult to solve once you said what the issue was. When I get told it's missing libebur128 and I can confirm easily that it does via build log and source, then I don't even know what the problem is to fix it.