Sigh. Another J4 issue. I just noticed the checked checkboxes in User Details are empty.
This appears to be a new bootstrap setting which I picking up from the Cassiopeia template.
It thinks it's checked even tho it's empty, and if I click on the Terms & Conditions box, I get the "This field is required." error.
...OK, it appears to be these lines for .form-check-input in the Cassiopeia template that empties the field regardless:
-webkit-appearance: none;
-moz-appearance: none;
appearance: none;
Some kind of conflict with CB's template...