If let's say the ECT sensor is reading higher at low temps the ECU will not start the glow plugs post heat operation, that's another chance but it can't be checked now as it's not cold anymore...watch through your logs, can you find at least one where the ECT is below 20*C
Most of the logs the coolant temp will be below 20. Perhaps it’s worth replacing the ect sender, they’re not that expensive.