After discussion on IRC (#debian-devel) I filed Debian bug 896506¹ about this failing command.
I'll continue to pursue a solution in this area. Maybe (suggested by josch):
--solver aspcud -o APT::Solver::Strict-Pinning=false -o 'APT::Solver::aspcud::Preferences=-removed,-changed,-new,count(solution,APT-Release:=/a=$unstable/)'
After discussion on IRC (#debian-devel) I filed Debian bug 896506¹ about this failing command.
I'll continue to pursue a solution in this area. Maybe (suggested by josch): :Strict- Pinning= false -o 'APT::Solver: :aspcud: :Preferences= -removed, -changed, -new,count( solution, APT-Release: =/a=$unstable/ )'
--solver aspcud -o APT::Solver:
¹ https:/ /bugs.debian. org/896506