After numerous complaints over flickering on external displays, Apple crafted a patch
for MacBook users that would supposedly fix this. It was deemed a software-fixable problem, and it only cropped up when using integrated GPUs as opposed to discrete GPUs. Unfortunately the patch seems to have made the problem worse
and, as a result, Apple has made the decision to pull the update for now, while they look into the true cause of the problem.
Complaints started to appear on Apple's boards post-update indicating that for some people the problem was still around, while others found it was amplified. Given that some people actually found the fix to work as designed, it is possible the problem is wider in scope than Apple realizes, with multiple types of hardware being affected. Apple is not yet sharing if it actually is a hardware problem or a driver/software issue.