Yes but as the OP is sailing from Barcelona, the practices in Copenhagen really don't apply. That's why I said it varies in Europe. In the past Barcelona B2B passengers could stay onboard. But I don't know that is what is going to happen this year so gave advice for either way.
I would say currently, all B2B cruisers in Europe do have to get off, whether it's Copenhagen, Dover or Barcelona, yes it occurred in the past. Security and procedures have tightened up in the last couple of years.
Can I ask, when do you refer to? What year?
I like you have given an answer from experience, mine is in Europe, this year, on the Magic, yes There has been one cruise to/ from Barcelona this year on the 1st where someone could update us on B2Bs I know a few who did Dover > Barcelona / 12 night Italian tour.
I would add " even" if they allowed European B2Bs to " stay on, a Barcelona round trip, then WBTA B2B would not be allowed. They will have to get off. This would be to satisfy USA immigration rules. A TA ending in the USA will require advance notice of cruisers to be advised to USA immigration, IE API and for non USA guests a ESTA or a Visa check, and the USA can instruct the port of Barcelona and
DCL to deny boarding, if they are not happy, IE prison record.
As the prior cruise is all in Europe is Not notified, but a TA that ends in the USA is notified, therefore anyone could be denied boarding, hence you have to get off, go through Checks and then get back on.
DCL can be fined if it doesn't co- operate or is lax. This has all been tightened in recent years.