What is interesting is that even the Study Model by Bill George seems to have this chasm.
The Blue Forcefield glow actually predated TUC (when alterations were made). Its appearance in TNG, particularly in Unnatural Selection reveals this blue glow:
So it seems there is a definite story here. I am very curious what Bill George envisioned with this opening.
Here is an early publicity shot shortly after completion of the model for TSFS:
ETA: Looks like misremembered the episode of TNG. That Excelsior shot from TNG was actually from the episode The Child, not Unnatural Selection. However it seems to have been filmed by ILM as part of the initial library of footage made for Encounter at Farpoint, dating this sometime around 1987.