Joe,
I never knew that regular bulbs would cause issues.
They don't.
I was told that they help the signal propagation.
They can.
This makes my basic fundamentals of DCS get erased and that's an overload right now!
At the risk of being accused of being less than a scientist, I will state what experience in the real world of DCS has taught me:
18 volt light bulbs, when used properly and appropriately with TIUs earlier than a Rev. L, are an excellent way to eliminate DCS signal issues, and to maximize DCS signal strength.
What a scope shows is irrelevant if your DCS works and operates properly.