Best guess? Whichever account gave account 6 permission to play their game.
Either account 1, 2, 3, 4 or 5 will be the user that gives 6 the permission to play their game, so it follows they’re the one that (I’m assuming) will get banned also. It’s a good question you raise and I’d be interested to know for sure myself.
2015 is when GTAV was released (for PC) Yet here we are almost a decade later and now they add anti-cheat…?
I don’t know what GTAV is like currently but the last time I played GTAV:MP it was a total mess. God knows why they didn’t think to add anti-cheat sooner… It’s not like Rockstar are hard up.