Odd error booking "stopover" flights

Anyone know why this error occurs?

I wanted to book an itinerary of AAA-BBB / BBB-CCC / CCC-AAA, where BBB-CCC and CCC-AAA are separate legs on the same day, with a several-hour stopover to allow for conduct of business.

Trying on AA.com gave the following error message for a paid flight:

Something went wrong
You can't book connecting flights as individual flight segments. Please change your flight search. Contact technical support.

When trying it using miles, however, the same itinerary was acceptable (and priced as 3 flights/trips).

Interestingly, trying to book the entire itinerary on Kayak gave no problems. It was possible to click through the link from Kayak to aa.com and book with no error message.

Likewise, booking only BBB-CCC / CCC-AAA (on the same day) caused no problems on aa.com for either paid or miles tickets. [Disclosure: I ended up booking this part with cash and AAA-BBB with miles, both on aa.com]

Any idea why the glitch for the full itinerary on aa.com?

Thanks!