I was checking issue #4325.
It is not reproducible for me (or OpenBrain) in Linux. However, two different french windows users can reproduce.
The problem looks like this:
So, it happens when creating an array like expression such as: Constraints[4]
User mario, identifies that the unsolicited character corresponds to the character that would appear with the combination of Alt Gr + the number used in the array, for example:
Alt + 4, ascii 4 ♦
Alt + 2, ascii 2 ☻
Alt + 7, ascii 7 •
Alt + 6, ascii 6 ♠
He indicates that the character appears just after releasing the Alt Gr key used for creating the closing bracket.
Maybe a non-french windows user could try to reproduce this (I say non-french because I do not know if the locale may be related to this behaviour).