As an aerospace software engineer, I would guess that, if this actually was triggered by some abnormal solar activity, it was probably an edge case that nobody thought of or thought was relevant for the subsystem in question.

Testing is (should be!) extremely robust, but only tested to the required parameters. If this incident put the subsystem in some atmospheric conditions nobody expected and nobody tested for, that does not suggest that the entire QA chain was garbage. It was a missed case -- and one that I expect would be covered going forward.

Aviation systems are not tested to work indefinitely or infinitely -- that's impossible to test, impossible to prove or disprove. You wouldn't claim that some subsystems works (say, for a quick easy example) in all temperature ranges; you would definite a reasonable operational range, and then test to that range. What may have happened here is that something occurred outside of an expected range.