Ok that concerns me, because that is where the powertank is not the contactor. That is further back towards the front of the seat. Although it might be heat from the charger that was rising over 7 hours and hitting the top of the tank plastic from underneath. i'll have to check it next time I charge from very low and see if there is heat like you describe. My bike sits outside however, so there is constant wind and airflow currents, even if just a little. I wonder if its nothing to worry about, but just because in the garage there isn't as much breeze blowing around.
If you feel it again, it might be worth sending your logs to Zero and just ask them to tell you your battery temp of the powertank on the dates and times you noticed it. I'm hoping the battery was very far from 50C and if it was, I think it might just be heat from the charger. If your powertank battery is regularly at 40-50C when charging and the monolith isn't, there might be an issue with the powertank. I hope not.
Whatever resistance would be causing heat in the powertank (loose bolt on the fuse or contactor) it would create much much more heat while riding vs charging. If its not hot after riding, I can be fairly certain its just rising heat from the charger then in a location without any airflow which means the heat will just go straight up causing the hot plastic tank you describe.
Follow up and let us know what you find out.