If you are experiencing issues with your headphones' Bluetooth disconnecting and reconnecting while on standby, there is a new Firmware Update app to resolve this issue. Start by downloading the app  "JBL Firmware Update" (see links below).
Google Play Store: https://play.google.com/store/apps/details?id=com.jbl.tune.update
App Store: https://apps.apple.com/gb/app/jbl-firmware-update/id1548227663

Once downloaded, you can update your headphones firmware from there. 
Below you can see step by step images of the update process.