Bill, I think we have a winner!! As suggested, I removed the overflow basin cover last night, ahead of another heavy (2") rain overnight. The pool drained as it should -- to the point where the water dropped just below the PVC riser in the overflow basin, and no more. Thank you for the thoughtful advice -- I would have never came up with this on my own. I'll be drilling those 1/4" holes in the lid later today.Does the cover have a hole in it like a skimmer cover? If not I am assuming when there is a heavy enough rain that is completely fills the drain pipe it turns into a gravity syphon. This syphoning continues till the seal between the cover and the box dries out enough to break the vacuum.
Simple test would be to leave the cover off when it's rains and see of it happens still. If that stops it, just drill a couple of 1/4" holes in it.
If it continues to happen I am at a loss.
Thanks again!
Bill