Just in case anyone stumbles on this, it’s an obscure but well-known problem with MBPs of about 2015-2016 build.
Apple are actually unable to fix it which is quite funny … incredibly even changing the motherboard (includes all ram and video cards as a fixed unit) does not fix it!
Any app (such as Unity - even just Safari) which calls for the faster graphic card, can cause the woe.
The only workaround is to use something like this https://gfx.io to force use of only the slow card. (Using the built-in “turn off both card use” switch does NOT help, bizarrely.)