Seems like that it's not that coding agents are to blame, its that the people who are ultimately responsible for committing and merging the offending code are to blame, regardless of its origin.
Or they are to blame because fixing 1000 CVE's doesn't magically absolve one of responsibility for regression bugs, even if one "accepts" them as a psychological salve.
If you are entitled enough then they are to blame they didn't fix everything at once, but in that case you really should be paying for their product and support. Otherwise fixing security issues has high enough priority to accept there might be downstream bugs that will be fixed in due course.