Some orders of imported component combination fail

[j-wags]

Reproduction
With input files from openforcefield/anything-goes#28

from openff.toolkit import Molecule, ForceField
from openff.interchange import Interchange
import openmm.app
import openmm
import openmm.unit 

inpcrd1 = openmm.app.AmberInpcrdFile('monomer.inpcrd')
prmtop1 = openmm.app.AmberPrmtopFile('monomer.prmtop', 
                                     periodicBoxVectors=inpcrd1.boxVectors)
system1 = prmtop1.createSystem(nonbondedMethod=openmm.app.PME, 
                               nonbondedCutoff=9 * openmm.unit.angstrom, 
                               switchDistance=8 * openmm.unit.angstrom,
                               constraints=openmm.app.HBonds)
i1 = Interchange.from_openmm(system1, 
                             prmtop1.topology, 
                             positions=inpcrd1.positions, 
                             box_vectors=inpcrd1.boxVectors)

inpcrd2 = openmm.app.AmberInpcrdFile('ALA_ALA.inpcrd')
prmtop2 = openmm.app.AmberPrmtopFile('ALA_ALA.prmtop', 
                                     periodicBoxVectors=inpcrd2.boxVectors)
system2 = prmtop2.createSystem(nonbondedMethod=openmm.app.PME, 
                               nonbondedCutoff=9 * openmm.unit.angstrom, 
                               switchDistance=8 * openmm.unit.angstrom,
                               constraints=openmm.app.HBonds)
i2 = Interchange.from_openmm(system2, 
                             prmtop2.topology, 
                             positions=inpcrd2.positions, 
                             box_vectors=inpcrd2.boxVectors)


offmol = Molecule.from_smiles("CCO")
offmol.generate_conformers()
ff = ForceField('openff-2.3.0-rc2.offxml')
i3 = ff.create_interchange(offmol.to_topology())

With this setup, the following orders of combinations have the following outcomes:

# Permutations of all three
#comb_interchange = i1.combine(i2.combine(i3)) # works
#comb_interchange = i1.combine(i3.combine(i2)) # works
#comb_interchange = i2.combine(i1.combine(i3)) # works
#comb_interchange = i2.combine(i3.combine(i1)) # works
#comb_interchange = i3.combine(i2.combine(i1)) # UnsupportedCombinationError
#comb_interchange = i3.combine(i1.combine(i2)) # UnsupportedCombinationError

# Three of the same
#comb_interchange = i1.combine(i1.combine(i1)) # UnsupportedCombinationError
#comb_interchange = i2.combine(i2.combine(i2)) # UnsupportedCombinationError
#comb_interchange = i3.combine(i3.combine(i3)) # UnsupportedCombinationError

Software versions

• Interchange main (after merging Fix edge case with combination #1418)