D2 Delta PX reconnecting iPhone does not work


I have problems with my D2 watch.

When I'm moving away from my iPhone, the Bluetooth connection will of course stop and showed as disconnected.

When I'm back into reach of the iPhone, the watch should reconnect automatically. This is working only sometimes!!!

When I go into Bluetooth Connection I can reconnect! But why does it not work automatically?

The iPhone is a new one and installed fresh without only old data. I have deleted the watch several times and did a new pairing.

My wife has a Fenix 5 plus and for her everything is working.

Does anyone has some ideas?