...doesn't sound like anyone did. From what I understand, the vehicles are not "off the shelf" and were custom designed/built.
My best guess is that whoever signed off on the design, assumed that ADA/Disabled folks could just xfer through the small door. Not the case and the Imagineers, who apparently have no disabled experience, had to engineer a way to covert a pocket sliding door, to a swinging door that (1) would not pinch the guest transferring (2) be wide enough and low enough to allow them [ADA riders] safe entry/exit and (3) still close securely so no one falls out. I joked they should just add handrails to the back of the ride vehicle and let ADA riders hang on.
This isn't "the" delay but it is "a" delay at both parks. CA has it figured out and is working on it - WDW, I have no idea. The 2 parks don't even share information with each other it seems.