The statement "In between two braces" makes sense only when we use vertical line cursor.
Yes I was referring to the vertical bar cursor because its the default cursor. Scintilla (and therefore Geany) operations are designed based on that cursor which is between characters. It is possible that behaviour that makes sense relative to a cursor between characters makes less sense for alternative cursors "on" a character but they are not the default.
Then it looks like the bar is touching the brace from the left or from the right.
Exact positioning depends on the font, since consecutive glyphs leave no space between them, Scintilla must "steal" some pixels from the glyph, so its likely that the cursor will look closer to one character or the other.
Without vertical line it is really unclear why the position after the brace is related to the brace, but the position prior to brace is not related.
As above, Geany operation is based on the default vertical bar cursor, so both the character before it and the character after it are equally relevant.
If only one is a brace, that is the one that is highlighted and used by goto. That is a useful feature, the cursor does not need to be moved if its the "wrong" side of the brace and its unlikely that removing that feature as you have done will be acceptable. The goto placement is such that another goto makes the cursor return to the original location.
If both the character before and the one after the cursor are braces the one that is before the cursor is highlighted and used for goto, which is a simple and consistent rule.