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 longer time in standby, you could try following steps (without warranty it'll help at all):

• some broken Adroid firmwares turn BT off during sound profile check even if the profiles are disabled: remove all do not disturb sound profiles in settings → sounds
• you may have some third party apps on your phone which disable BT, for example apps like “Tile” often reconnect BT in background
• here are more suggestions that can help >>

If everything above fails, you may need to use display always on as a workaround. You can reduce the screen time if you turn your display on each hour right before the BT connection goes to sleep:

1. Install app Tasker
2. create a loop of 1 hour waiting every time
3. add a turn display on action (Android 5+ needs an additional tasker plugin app “Secure Settings” to get access to add task > plugins > secure settings > wake device > screen on)
4. add a notification wake display %TIME to confirm your workaround is running
5. optional - if you disable lockscreen, start a touchscreen lock app to prevent inputs during your screen on action
6. optional - add a display off (display lock) action if you don't have any display timeout

This prevents your Android device from entering any deep sleep states but drains way less power than when display is always on.

For more information, read on in this >> or this helpdesk thread >>