It's in the firmware
Filament runout sensors generally trigger M600. Usually that preserves the bed temperature, but some firmware distributions might have this set up wrong.
How is M600 working?
The firmware dictates what is done in case of filament runout. The standard settings in configuration.h can look like this:
There is no easy way to keep the bed heatred during pause
Looking at the sourcecode, I am pretty sure that the current version (10/29/2019) keeps the heated bed at the right temperature.
Features are built into the firmware. You need an Arduino and some wires to write new firmware to the built-in SOC. Adjusting the code should be as simple as commenting a ...
3D Printing upside would only potentially have an issue with the first layer if you're using an extremely large gap on the first layer, however in normal circumstances there's enough pressure that the filament is squished into the bed, if you've got one yourself you can put it on it's side, the question when it comes to 3D Printing isn't upside down, ...