The issue occurs when the Master-1 script is running before the Master-2 script.
After connection is established with the Master-1 the DUT sends L2CAP Connection Parameter Update Request. If the Master-2 connects and starts sending Connection Update Indication at this time – the DUT is stuck.
Notes: the Master-1 script only establish connection to the DUT, while the Master-2 script after connection runs loops of Connection Update.
The issue is not reproduced when the DUT runs the Simple_Peripheral app.