I use LibreOffice 126.96.36.199 under Windows 7 Home Premium 64bit (German language). When I try to type special characters like brackets via pressing CTRL + ALT + one of the number keys (German keyboard layout), nothing happens. The problem does not occur in Calc, Impress, nor the Windows text editor. It also does not occur under Ubuntu 14.04 nor 16.04, so I think it is a problem of LibreOffice unter Windows.
*** This bug has been marked as a duplicate of bug 100908 ***