I'm not a developer or using an emulated BT keybaord, but had issues with Bluetooth on my MacBook Pro (Retina, 15-inch, Mid 2015) when I installed 10.15.3 installed yesterday. None of my BT devices (Apple KB, Trackpad, Magic Mouse) connected. After rebooting once, I got the mouse and trackpad to come back intermittently, but not the keyboard. I removed devices, reset BT module, reset devices via the Bluetooth Debug (Shift-Option + click on BT in menubar). Didn't help. Reset NVRAM and SMC. No help. Called Apple support and explained. Tech had me boot into Safe mode and I was able to see/connect all BT devices. After restarting normally, all BT devices are working now. The tech explained that the Safe boot can clear some caches which may have fixed things. Worth a try if you haven't tried the Safe boot. Mike

Sole too soon. Problem is back, even in safe boot. Time for another call to Apple. Sigh.