I dived more into the topic and found out following things:
- While lobbies can theoretically be used as parties, EOS implementation of them doesn’t allow to update custom parameters correctly, thank you Epic Games again (eg you can set session settings custom parameter on host to enter matchmaking queue, a delegate about session data change will be fired on clients, but custom parameter will be missing if advertisment via ping and only first value, no future updates, if via online subsystem; UPD: resolved this issue here).
- Sessions custom parameters in session settings will work, but I want to save them for matches, not for parties, because dedicated servers can’t create lobbies
- Anyway joining or creating lobby / session with OSS will trigger map change by the engine, which is completely undesirable in some cases, like parties
- I won’t be struggling with Epic Games ■■■■ anymore, as they don’t care about us making parties in our games via their online services, I’ll just make a websocket implementation for parties myself.