After new players joined my server, they didn't get assigned to default group, I've checked with listmembers and they weren't there. I had to manually add each one of them to the group. Am I missing something?
I'm using the 5.0.72 version for my Spigot 14.4.4 server.

Yes. Players added automatically to the default group that are not online don't show with listmembers.
You can use /lp user <user> info to verify that they are in the default group.

@BrainStone "Online players" mean players that are connected with a Mojang account right?
I tried your method and it works. Thank you.

I mean players that are on the server.
There might be a chance that they then still don't show there.

@BrainStone Well the player was on my server and listmembers didn't do it, but when I manually added them to another group, listmembers would show their name.

I see. Then listmembers does exclude all players that are automatically assigned to default. There's a technical reason for that.

Basically everyone who doesn't have a group is in the group default.
No database entry is created to be more flexible and use less space in the database, etc.

Therefore there is no way to know who is in the group as there could be million members, but they aren't saved anywhere.

Yeah this is normal :)