The game itself should never be able to go into Portrait Mode.
However, I wonder if that could possibly be due to your device going into Multi-Tasking (and actually multiple apps open on the same screen at same time) ?? Maybe device is stacking 2 or 3 apps on top of each other (on a Phone/Tablet's Portrait held orientation), with each of the 2 or 3 open apps (or at least, this Game) being in a Landscape mode within their multi tasking/view portion of the screen.
Or even if it is just the game that appears open, device could have switched that into a multi-view mode (without other Apps being in those other view windows), making it look like game is in Portrait mode ??
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However, I wonder if that could possibly be due to your device going into Multi-Tasking (and actually multiple apps open on the same screen at same time) ??
Maybe device is stacking 2 or 3 apps on top of each other (on a Phone/Tablet's Portrait held orientation), with each of the 2 or 3 open apps (or at least, this Game) being in a Landscape mode within their multi tasking/view portion of the screen.
Or even if it is just the game that appears open, device could have switched that into a multi-view mode (without other Apps being in those other view windows), making it look like game is in Portrait mode ??
got a screenshot of it