Ask the choir to get aggressive with their releases.
Rhythmically accurate entrances are much more easily attainable than accurate releases. Young singers just seem to want to…arbitrarily stop the sound at the approximately correct spot.
Particularly in rhythmically active music, and particularly when you can’t add a shadow vowel (say, an open ah) to clarify the release, it’s time to get aggressive.
Rhythmically accurate releases can dramatically improve the quality of the performance, and yet they are often overlooked in rehearsal in favor of other musical elements.