1) Classic has its own Bug Report Forum.
2) What do you get back when you do /dump GetModifiedClick("SPLITSTACK")? If the split stack option is correctly bound to the shift key, the output of that line should include something like [0] = "SHIFT" (format from memory, but something about shift even if not exactly that line).
If it isn’t properly set, you can use the SetModifiedClick command to change it back.
/run SetModifiedClick("SPLITSTACK", "SHIFT")
The other options for item links, socketing, etc can be found in that link.
Oh, and make sure to save your keybinds afterward if you do run that set function.