root now has ExcludeArch: %{arm} s390x on rawhide, which makes this package fail to build and install on both armv7hl (and s390x, which is known). The BuildRequires on root have been conditionalized for non-s390x builds, but it looks like %{arm} now needs to be excluded as well.
The unavailability of root on 32-bit ARM was temporary due to a problem with the compiler. It is now available again after having been compiled using gcc-10.0.1-0.9. No need to change the HepMC3 package.
Awesome, thanks for the update.