This has always been the case. If your battery is more than about 2/3 full, it cannot charge fast enough to handle more than B2 levels of regen. To compensate, the "waste" the regen power by having the generator spin the ICE (without injecting fuel). They don't want this done on a cold...
Why would removing the heater increase range? In most weather it wouldn't be used, and would have no impact. I don't think range certification is done at - 20c.
The point Youngfeller was trying to make is that by using "charge" mode you burnt a bunch of gas to generate electricity, which you then stored in the battery (rather than converted into forward motion).
If you didn't end your test by driving the battery down to zero, your calculated fuel...