1. apply all instructions described here >>
2. especially disable battery optimization for bluetooth system services as is described here >> (at least for Android 6 and 7 on Sony and Samsung devices) - you may need to toogle show all system services. Disable battery optimization for following services:
   • Bluetooth MIDI Service
   • Bluetooth Sharing
3. clear cache and remove data of Android Bluetooth system services - settings > apps > 3-dots → show system processes:
   • Bluetooth MIDI Service
   • Bluetooth Sharing
4. restart Android and the issue should be gone in most cases

If you still have issues with loosing BT connection during long time in standby, you could try to do the following steps which are less likely to fix anything:

some broken android firmwares do disable BT during sound profil check even if they are disabled: remove all do not disturb sound profiles in settings → sounds you may have some third party apps on your phone which do disable BT for example apps like “Tile” does reconnect BT in background maybe other suggestions could help you https://productforums.google.com/forum/#!topic/phone-by-google/UamV0tnZKNE And if everything does fail and you need to use display allways on as workarround. You could reduce the screen on time if you turn your display on each hour right before the BT connection go to sleep.

Install app “Tasker” create a loop which wait 1 hour every time add a turn display on action (Anroid 5+ need an additional tasker plugin App “Secure Settings” to get access to add Task > plugins → secure settings →wake device >screen on) add a notification “wake display %TIME” to tell you that your workaround is running optional if you did disable lockscreen, start a touchscreen lock app to prevent inputs during your screen on action. optional add a display off (display lock) action if you don't have any display timeout This will prevent your android device from entering any deep sleep states but drain way less power then display allways on.

If you have BT problems even with display allways on, you have maybe an android hardware problem, environment problem or BT transmitter problem. Check if your BT transmitter is causing the problem. Some devices stop sending data on fireware specific conditions. If you don't have any issues with track recording or sometimes the connection got interupted by your testing environment. (hardware issue or signal interferences. Try to make a track recoding with screen always on in a controlled static envirnoment with less wireless and bt interferences as possible.