So, does this happen with all maps and only in online multiplayer games?
If the problem occurs on a specific map, please share the map file if possible.
If it happens on all maps, try typing "@CommonStart()" in the console after loading the map (even if it shows an error). This should resolve the issue, at least for now.
Also, is the first player set to AI by any chance?
In any case, thank you for the information; at least now we have a better idea of where to start looking for a solution.