The definition for a PDESystem seems to show that internal systems are compatible, (PDESystem · ModelingToolkit.jl) but it looks like discretization of the PDESystem does not look at these systems, causing errors when discretization is attempted. From what I can tell, the equations, variables, and parameters from these internal systems are not pulled into the main PDESystem in the way that they are for ODESystems on system definition from eqs, ivs, dvs, etc.
ODESystem(eqs, t, vars, pars; systems = [sys])
vs
PDESystem(eqs, bcs, domains, [ivs], [dvs], pars; systems = [sys])
Are internal systems (e.g. from ModelingToolkitStandardLibrary) intended to be compatible with the PDESystem definition/discretization, or is this unsupported? The only reference to PDESystem.systems that I can find is in Give PDESystem a systems field by xtalax · Pull Request #1588 · SciML/ModelingToolkit.jl · GitHub which seems to be related to visualization of equations than added functionality.