In our case we knew we needed boosters for Europe and had them, but when we did the Checkin we could only put the original vax card and couldn’t show the back that had the boosters. When we boarded we showed them the back and thought all was well.
it wasn’t. The day before we got off the ship our card was locked to any further charges and it was because at the start of the cruise the agent never recorded our booster. Thankfully guest services straightened it out.
I think now you’re able to upload more than 1 photo so you can show boosters.