Yes Tristan and all the others were all originally located in another country and had apartments in that country. Artem moved to a new country and a few months later Tristan joined him there. The others moved to a different country.
After Artem left the project, Tristan found it difficult without his friend and decided he would be happier living closer to all the others that he was friendly with and so decided to move. So although originally they all resided in the same country, the country that Tristan arrived from last night was not the same one that Roza, Derek, Gina etc. arrived from.
As I said earlier, I don’t know if this was a factor in what happened last night, it’s just my speculation based on a known difference between Tristan & the others. It could just be that the person on border control when Tristan arrived was in a bad mood and applied the rules more strictly.
It doesn’t really matter why Tristan was refused entry, it doesn’t change what happened. I just hope that Tristan can find the happiness in his current location that he was hoping he would find with Roza & co.