I had that too last week, took about 20 minutes to finally clear so I could rebook at the cheaper price. I was on edge thinking I'd lost out on the excursion whilst trying to save some money. Didn't do that a couple of months ago
Last week went on 4 night cruise on Ventura, had flexible dining - dining app was not in use. We tend to eat early and had no issues with dining on a table for two and no waiting
It was fine as we were a family group on a table for 6, also it was a round table so there was distance between the tables, waiters were the same throughout
Interestingly Southampton Cruise Ship Schedule has Britannia arriving at 06.15 and departing 22.00 on 1st September (friday). Perhaps late departure in order to check thoroughly and repair