Update: I had previously backed out KB4515384 on my primary because of the Start Menu search bug, but it was still installed on my secondary. Since backing it out on the secondary yesterday, I have had no disconnects.
The only issue I still have occurs when the secondary has been inactive long enough for the monitor to shut off. In this case, when I drag my mouse to the secondary, there are several seconds of latency where the mouse is non-responsive. Even if I drag it back toward the primary, nothing happens. Using a hotkey to jump back to the primary does make the mouse responsive immediately. If I take no action, everything is fine after a few seconds. I've experienced this well before KB4515384 was published and am just treating it as a minor annoyance.