In the last hours we have received communication from Apple that their new update, iOS version 13.4 could cause Bluetooth connection problems, as highlighted in many other products:
https://forums.developer.apple.com/thread/129969
https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/219245/ios-13-4-bluetooth-connection-problem
https://www.forbes.com/sites/gordonkelly/2020/03/26/apple-ios-13-4-cellular-data-security-iphone-11-pro-max-upgrade/#8462a522f750
D-Heart could also be affected by this. Although we expect to solve the problem within a few days thanks to D-Hearts technicians. If you want to avoid a disruption in D-Heart’s usage, we suggest you don't upgrade for the moment until any problems will be resolved.
Update 05-21-2020:
We are finally pleased to announce you that new iOS software update (iOS version 13.5) is now available on the stores. With the present version Apple solved all the connectivity bugs present in the 13.4 release, therefore allowing perfect use of many Bluetooth connected devices.
We are also happy to announce you that we will be releasing two major D-Heart App Updates, both for iOS and Android, aimed at improving user experience, several important bug fixing and new functions such as the telereporting personalization.