This isn't actually how Apple work, teams aren't really siloed.
They are.
See also: why is the 2021 iMac a complete redesign but doesn't feature Center Stage, something introduced at the very same keynote?
Answer: because the iMac and iPad Pro teams were too silo'd for that to be practical.
More rigorous software testing would not create this problem.
"If people made fewer mistakes, fewer problems would occur!"
Weren’t there security vulnerabilities in 15.4, prompting the update to 15.4.1?
Yes, but I imagine the impact is rather small on a display that has no networking (neither wired nor wireless). Maybe if you connect a USB stick to it? But even those are mostly just pass-through to the host.