I do not have a custom shortcut.
I did not know that the R studio has not short cut [Control] + [Shift] +[;].
Since [;] =[+], this short cut is same as [Control] + [Shift] +[+].
And by execute this short cut, the font size in the R console becomes bigger.
I am not sure, but my key Keyboard layout is JIS, so not US, and this cause this odd behavior?