" if qt6-qtsvg is present at build time." I don't package for Arch, and noticed the same issue as reported here. As a packager you are not supposed to hunt through all commits when a new release comes out, to see if you need to add new run/build depends, that is what the CMakelist is for.
Looks like this commit: https://github.com/rncbc/qjackctl/commit/280dc2136c4c4a8192c935a5081347ee4640a6af missed adding the need for having qt6-svg. Nothing in the build or cmake options mentions needing this, thus imo, not a packaging issue, but a missing build setting in qjackctl, see the list it sets as needed now: https://github.com/rncbc/qjackctl/blob/master/CMakeLists.txt#L140
Rosegarden 20.06 needs Qt 5.15 patch
Build fix for Qt 5.5
Simplify .desktop install