Questions about Bosch Home & Garden Indego 400 Connect

I installed the robot with perimeter wire as per the manual. It works perfectly but in the centre of the lawn it loses the connection with the perimeter wire (I don't know if this is by coincidence after updating the software). I have also tried changing the wire ID, furthermore to be on the safe side I have replaced all the wire (about 1oo metres for a surface area of 260 sq m). There are no obstacles and it does not receive within a 6sqm radius with distance to the perimeter wires of 4-5 metres, the ground is flat. Do you have any suggestions or is it a defect in the device?