The character appears after the quotation mark
The qoutation mark disappears. Instead white Hex-Numbers in black blocks appear and the new character is placed inside. For example, if I use a german opening quotation mark („) and type the letter (uppercase) D, i get [xE2]D[x80][x9E]. Note that the hex numbers together form the UTF-8 encoded „.
The problem can be worked around by clicking with the mouse after the (replaced) character or by pressing the left-arrow key. That leads me to the following suspection:
I suspect that if you enter n characters in the quotation mark textbox, when replacing such a quotation mark, the cusor position is incremented by n bytes instead of characters.