It’s a bit surprising that it would respond to the ReadScratchpad (with an 85°C) but not act on the preceding SkipROM and ConvertT commands. If it were a hard wiring fault, you’d expect to get back DEVICE_DISCONNECTED (-127°C) although there’s a bug in some sketches that will cause that to get reported as 25.5°C. (https://community.openenergymonitor.org/t/get-temperature-can-return-a-bogus-25-5c/593)
Another possibility is that there’s a voltage sag during the conversion, severe enough that the sensor resets and so then responds with it’s power-up state on the ReadScratchpad command.
Are you running all 3 wires to the sensor (Vcc, GND and 1-wire)? How many sensors do you have on the bus? How are you powering the emonTx?