This is a known issue by Novation and logged with support - didn't want to lose sight of it in the new forum.
Temporary work-around is:
make sure the lock button on the Nocturn is not lit and then open Automap to the mixer/plugin mappings page and open the Channel 1 mapping. You'll see the buttons toggle between transport controls and normal buttons when you press the lock button. With this unlit, please assign each button to a different CC number and set the button type as Normal.
Now in Ableton, if you click the MIDI button you can learn CC messages to Abletons functions. Once you've pressed the MIDI button, click play in Ableton and then press the play button on the Nocturn (still with the lock light unlit). Do this for stop and record and then again press the MIDI button. Now you should find that the buttons control your transport. Make sure the buttons are assigned as 'Momentary' for play and rstop and 'Normal' for record.
Support have requested a fix from development however no date has been shared to when this may be fixed.