Bluetooth connection gets broken on iPhones iOS12

I am browsing the forum and other websites, but cannot find a similar problem I have, let alone a solution.

My problem is that I have a Garmin app that regularly polls an internet page to upload a bit of data (typically 1Kb json data). Parameters are being updated so memory size stays the same. This app worked flawlessly until recently (last few months). When the app is running for a while without any problems, it can happen that the bluetooth connection gets disconnected and no data uploads are possible anymore. I suspect that this happens more frequently for iPhone users with iOS12. When I switch off and on again bluetooth on my iPhone or Garmin watch it is OK again and bluetooth connection is repaired.

Is this problem known? Can I do something in the software to solve this?