You are correct. It is shockingly bad and rips you out of the being in dark cave moment. The light leakage is bright enough that the you can see unthemed backstage areas - which makes it almost certain that rockwork, a wall or curtain was removed. In this past, this area always felt like a dark cave - I'm a stickler for this kind of stuff and would have definitely noted egregious exit signs. In general, Disney tended to be much better about exit signs in the past (also hampered by their legal dept, nutty fire marshals and litigation-obsessed culture - e.g., they added plain, Target-bought exit signs to the outdoor red canvas-covered queue at Haunted Mansion... really...)
As you state, something was changed at that spot during this refurb (curtain or wall removed) and it sucks.