Screen disappears after couple of seconds on F45 or Vivoactive4 (due to battery optimizations)

In 10-50 seconds, a screen turns black in FR45 or Vivoactive 4 when it it is not redrawn every single on-update. Simply no-operation in onUpdate will turn the screen black instead of keeping the last image. 

No problems reported on other devices. 

Reason for not redrawing the screen all the time: I strive for battery saving and don't render seconds. It takes a lot (time-wise) to compute all charts and redraw the screen every time. So I redraw it only once a minute and I had a magnificent battery life. Better than stock watch faces. 

How to fix(?):

1. can the problem be solved by updating the firmware of the affected watches? 
2. is there a way how to tell the watch just to keep the current display

How to measure a battery impact(?):

1. how to measure the battery impact empirically? I just measured a rendering time in ms, but don't know what impact on the battery it actually has. Is there a better way to measure and optimize the battery impact? 
2. what operations actually drains the battery? 
3. Does BufferedBitmap count to the watch face memory limit? If not, can prerendering into it can improve a battery consumption? 

Thanks for your guidance and fixing the affected devices firmware.