Hi John,
I'm sorry to say, this looks to me like a defect.
When you combine signals, the resulting signal should get properties that exist on ALL the original signals (with the same value). So if even one of the original signals (ie before combination) has a different value for the property (or no value at all) then the combined signal won't get the property and the constraint would fail. However, I've just tried this and it looks like even if ALL the original signals have exactly the same property value, the combined signal does not. It seems to work with attributes, but not with properties. I would be grateful if you could report this to your local support team as an SR so that we can fix it.
As for workarounds, the best I can see at the moment would be to edit the name of the combined signal and then match by Name.
If for some reason you can't do that, a custom constraint could be used (because it could navigate to the 'source' signals and check the property there).
Regards,
Simon