We were on an Western/Eastern Caribbean B2B in 2017 on the Nieuw Amsterdam. We did anytime dining and usually showed up at the MDR around 8:00. The first week was great. The first evening of the second week, we showed up at our accustomed time and saw a long line on the "reservations" side of things. We were informed that a big group had the dining room and anytime dining was unavailable. The staff seemed to be uninformed that there would be a group monopolizing the dining room. Had to feel sorry for them. Felt sorry for us, too. At that time of night the Lido was closing down and our only choice would have been pizza. Eventually they found a table for the six of us who were waiting to be seated, and they got us a two-top in fixed dining for the week. As I recall, the group had a mass gathering earlier in the evening and were all dismissed at the same time for dinner. I believe the evening show was offered only once at around 8:00.
We definitely check the Charter/Group Information thread when booking now, and usually book cruises longer than 7 days.