There seems to be a dead zone, I've noticed that by turning on and off spartan and bartender, alternating, the dead zone seems to move, but not disappear completely. Unsure what may be causing this, but it did disappear when I turned both off and was on the native WoW UI.

If you type /sui move is there something where the dead spot is? Also please make sure you are fully updated.

I know you have only mentioned SUI and BT4. But do you have any other addons active? It is possible that another addon has a custom frame that SUI's frame manager doesn't recognize and therefore doesn't show. Modified frames should show using the frame manager since they are just altered WoW frames...but new custom frames may not, and will likely only be moveable by that addon's options. An example of this would be combat logging addons like skada, recount, details, omen, ect. If using a frame mover, the frames for those addons don't show up since they are custom frames. You have to use those addon's settings to show/hide/move them. And if a frame like that is for some reason hidden and not set to click through, then it will cause exactly the issue you are describing.

Type /fstack in your chat this will enter a special mode that says what is under the mouse. mouse over the deadzone and hit alt this will toggle between items under the mouse, take a screenshot when you find the item that is causing the deadzone and post that screenshot.