Ah, my bad, I figured that meant the device was using Cayenne LPP, hence I linked to that protocol. But the payloads you’re showing are not LPP.
Given the port number 103 and the the other decoders that @iiLaw linked to it seems Tabs might always be sending the temperature value as 7 bits and with an offset of 32:
// Board temp measurement
temp_board = bytes & 0x7f;
temp_board = temp_board - 32;
For your payloads, hexadecimal 0x42 would then decode to decimal 34, 0x41 to 33 and 0x3D to 29. So, that would work. But as your payload is only 8 bytes long, maybe that decoder simply does not apply here. Still, based on only this, it seems the sensor’s payload is okay so something is wrong in your myDevices setup.
Without knowing what values were displayed (and expected) for these specific payloads, it’s hard to help. I don’t know how things are decoded in myDevices, but to allow others to help you, I’m quite sure they’ll need many more details about your setup in TTN Console and myDevices. Also, the exact device type, any maybe even a link to its documentation might help.