I think I must have set it up wrong.
I went to a screen to set up automatic overdraft from my brokerage core account to my cash management core account. It asked for a couple of specific values. One was at what level do you want to initiate a transfer, and the second was how much do you want to transfer.
It sounds like the non-confusing way would be transfer "what's needed" when the balance is "$0", but the screen I was on didn't seem to enable the "what's needed" part. It's obviously possible, since several posters have it set up that way.
I had it transfer $4,500 when balance went below $300. So the confusing bit was when the balance was, say $400 and then a transfer of $1000 came through. The $400 would get transferred right away, and then the $600 the next day. Not impossible to understand, but a bit unsettling. And the weird thing with two checks getting written...one delivered, and one returned. Just the words "returned check" grate on my nerves, even though it had zero impact to my finances.
But now that I've taken 100% of my FDIC funds in the cash management core account and bought SPAXX, I think I'll not have any of those weird occurrences.