The Starliner issues are about nitrogen tetroxide flow

I haven't been paying attention to the details of NASA's Starliner saga (other than to note the unfortunate Boeing connection), but I did not know that the core issue is a chemistry problem: 

NASA's concern about Starliner's thrusters boils down to the failure of five of them during the vehicle's ascent to the space station. Starliner's flight computer shut off five thrusters, provided by Aerojet Rocketdyne, in flight. Four of the five thrusters were recovered after overheating.

Since then Boeing and NASA have conducted ground- and space-based tests of the small thrusters to try to replicate the failure and better understand, fundamentally, what is occurring. By getting to the root cause, the engineers will feel confident in their ability to address the problem for Starliner's flight back to Earth.

In ground tests, the engineers were able to demonstrate similar failures. Subsequent inspections showed bulging in a Teflon seal in an oxidizer valve known as a "poppet," which could restrict the flow of nitrogen tetroxide propellant. The thrusters consume the nitrogen tetroxide and mix it with hydrazine fuel for combustion. Despite the tests, however, engineers still don't understand precisely why the bulging is occurring and whether it will manifest on Starliner's flight back to Earth.

"People really want to understand the physics of what's going on relative to the physics of the Teflon, what's causing it to heat up and what's causing it to contract," said Steve Stich, who manages the Commercial Crew program for NASA. "That's really what the team is off trying to understand. I think the NASA community in general would like to understand a little bit more of the root cause."

I wonder what data they have on the material compatibility of nitrogen tetroxide with Teflon? It's probably pretty reasonably corrosion-resistant but I dunno, space is weird. Hmmm.