Same exact thing happened to us on the Symphony of the Seas a few years ago in the Mediterranean. Beach bed in solarium had book and towels on it forever so we finally just took it and put their stuff on a lounger next to it.
They came back over an hour later and started yelling at me saying they had gone to lunch at the buffet, blah blah blah. I just layed there and ignored them. Then they went and got a pool attendant and came over and I said “oh there was nothing here when we got here, sorry”. Then some nosey Karens said “they (we) moved your stuff”. More yelling ensued, I continued to ignore them and they eventually went away.
You don’t get to go to the buffet and eat a leisurely lunch and hold your spot. Yes I can see running to the bar or running to grab a hot dog or burger, but not a sit down lunch on the opposite end of the ship!!