In theory of course.
EU Border staff such as in Spain, can ask to check that you have enough spending money, a return ticket / onward ticket as well as all the passport checks ensuring it's in date, valid during your time in the EU and that you haven't been in the EU too long.
I guess they aren't doing this in full - but they have the right to. It's what we should be doing at UK immigration. Just so we know that people coming here can stay and aren't trying to sneak in for work.
This is what being a 3rd country out of the EU means. Before hand, at ferry ports, we were waved through or a brief scan. Potentially these checks will be time consuming.
And if the EU or any country in the EU wishes to make a point, this is what they can do.
www.msn.com/en-gb/travel/news/spain-introduces-new-85-rule-for-british-holidaymakers-entering-country/ar-AAZVRVQ?ocid=msedgdhp&pc=U531&cvid=dde0bb90bf474bc3ad00ae5097ea7424