I had a very similar experience a few years ago with Tannenberg. An eastern front WW1 shooter that, at least at the time, I don't know the current status, had just enough players in the evening to fill up one server, so I'd play with the same people night after night. It never felt empty because of that and it was great fun.
Isonzo is the newer one. I haven't played it in a few months, but it's similarly small but I never felt close to anyone there.
I play Squad fairly frequently, and it's got a similar feel to what the OP is about. You choose your server with a server browser, and it's frequently got a lot of the same people there all the time. There's some servers that are more casual, and they end up cycling players more so you don't recognize anyone. The more experienced focused servers draw from a much smaller group though, and they play more consistently.