I'm currently on Meteobridge 4.2 (Oct 22 2019, build 13222), FW 1.4 but I saw this with an earlier version also. I'm not sure of that version because I didn't check before rebooting.
Anyway, this either used to work correctly or I'm hallucinating. I modified Ken's Saratoga template version of gen-MBtags.php back in July to include the th0heatindex sensor. The unit-of-measure for temperature is set (and always has been) to °F and that's initially what I was seeing for heat index numbers.
But now, I'm getting all of the heat index numbers in MBtags.php in Celsius. Temperature and dew point are still in °F.
Any chance something changed in the firmware to cause this? Is this intended behavior?