I'm sure they do test, and a regression like this one would be a "test escape" that should trigger a review and new test method to catch it in the future.
I'm not a Tesla insider, but I work in software development. Requiring software to be 100% bug-free is unrealistic, but rapid iteration can get fixes out more quickly.
Whichever way you cut it I think many car manufacturers (not just Tesla) are wrong to tightly couple software iteration with hardware functionality.
It’s a car, not a software application. Properly tested, paid for and working features should come before agile software development. If that means it takes longer to release then so be it imo.