Aha! I just remembered that there's three JS includes at the bottom of the page code, and one of these clashes with the JS include used/created by DataAssist!
<script src="//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script> created by DataAssist
<script src="js/jquery-3.2.1.min.js"></script> used by bootstrap 4
I have tried removing the earlier version created by DataAssist, but the date pop-up still doesn't activate. Obviously, taking out the later BS4 version has other ramifications!
Is this a bug that needs fixing? Have I eventually found something wrong with the beta? It's been pretty bulletproof for me up until this point!