When you Launch Minecraft on Full Screen and you go to settings> Video> you can see the button of Full Screen keep on "Off" but it is "On", this just be when you close MC with this option activated.
Maybe this is not so important but I report it.
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Thank you for your report!
However, this issue is a Duplicate of MCPE-34361.
If you have additional information, please add it to that report.
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