Also having the issue. Intermittent flickering when in slideshow mode.
MacOS Sierra 10.12.3
Powerpoint 15.31 (365)
Macbook Pro 13" TouchBar
The way I've been temporarily stopping it is by adjusting the brightness, as others have also suggested, which is unacceptable in the long term. Other suggestions such as reducing resolution and closing the lid are even more inconvenient and unacceptable.
Also tried a number of things on the laptop end to try and resolve, as suggested by apple support, including NVRAM and **** reset, running diagnostics, running in recovery mode, reinstalling Sierra, wiping the laptop and installing Sierra and applications
from scratch.
I believe I submitted the feedback via the smiley when I first experienced the issue a couple of weeks ago, but have just done it again to make sure.
If you are experiencing this issue, please submit the feedback via powerpoint to make sure it's sufficiently on their radar and that they are aware of how widespread the issue is.