OK, since you asked: Why did their membership in a union not preclude this outcome, given that one of the purported benefits of unions is to prevent such outcomes?

This is like asking "why did greenpeace not preclude climate change".

Because that behavior is widespread across the whole industry, across many countries and across 4/5 decades:

https://en.wikipedia.org/wiki/Defeat_device

https://en.wikipedia.org/wiki/Diesel_emissions_scandal

Unions, just like any other non-governative organization, are just as powerful as people support them.