One of the unforeseen consequences of installing iOS 13.4 and iPadOS 13.4 was that FaceTime calls may not work if you were FaceTime-calling someone who had iOS 9.3.6 or earlier versions of macOS, specifically El Capitan 10.11.6 or earlier. This has now been fixed, Apple says.

Choosing Bluetooth from Quick Actions
There was an issue with the Settings app where, if you wanted to choose Bluetooth from the Quick Actions Menu, the one on the Home Screen when you swipe down from the top-right corner, it might not do what it should. With 13.4.1, everything should work just fine again.
iPadOS 13.4.1 flashlight issue
This sorts an issue on the most recent iPad Pro models. That’s the fourth-generation 12.9-inch and second-generation 11-inch iPad Pro. Tapping the Flashlight button in Control Center or on the Lock screen could result in nothing happening. Which, obviously, wasn’t the plan. This certainly wasn’t universal, I never experienced it, but this update should fix it.