@AVsetula Thanks for your reply. To help troubleshooting, I made up an RJ45 connector breakout and viewed the 'data out' pin on an oscilloscope. It soon became clear that the problem is one of intermittent connection. I connected the scope at the end of the device chain (socket 8 on the hub) and if I connect all the intermediate sensors, and tape their cables to the bench so there is no strain on the RJ45 connector, all is well. I can see nice sharp 0V to 5V data transitions on the scope.
But, any movement on the cables causes an intermittent loss of contact, immediately apparent from the 'illegal' voltages that appear on the scope display.
I have disassembled the hub and buzzed out all the solder joints: no problems. The issue seems to be either with the RJ45 plug/socket connection, or with the sensor cable/RJ45 plug crimp integrity.
It doesn't help that the insulation on the temperature sensor cables is extremely stiff, so that any movement of the cable puts strain on the RJ45 connector.
I think I will need to arrange for cable clamps to hold the sensor cables in place. The hub will be mounted in a plastic box, so this is feasible: I should be able to 3D-print something.