Budgies are from one of the hottest countries on the planet. Therefore you would expect that they could survive extremes of temperature, and they do.
I have noticed that a lot of breeders loose birds in weather that I would describe as moderately hot. My region has temperatures that regularly climb into the high 40's the top temp last year was 48 degrees C. The birds survive and actually breed fine without the need for air conditioning etc.
If you are loosing birds I would check a few things:-
Good Luck and Stay safe.
All the best for 2020.