Just to clear a few things up…
If you have immunity to measles, either via vaccination or previous infection, you’re extremely unlikely to catch measles again, and therefor extremely unlikely to make anyone else sick either. It’s the folks who are younger-ish, unvaccinated, not previously infected, and who get exposed when traveling that bring measles back to the US who have traditionally started the mini (to now larger) outbreaks. Remember the incubation period for measles is normally fairly long, 10-14 days, which is how you can get infected when traveling, but not sick until you get home.
Also, a negative titer test does not mean you’re not immune, it just doesn’t confirm you are. It is possible that the body’s cell mediated immune system (B and T cells) still has all the “records” it needs to produce more antibodies if you’re exposed, but isn’t doing so because you haven’t been exposed. Catch is you don’t know what a negative test truly represents.