It seems like grouping /list items is only possible with the first entry.

list:
    # To merge groups, list the groups you wish to merge
    #Staff: owner admin moderator
    Staff: Tech Admin smod mod
    Helfer: Helfer
    Legacy: Senior
    Spieler: Spieler Voteaktion Votearmour Votemmo

The Staff grouping works well.
But players in the groups Voteaktion, Votearmour or Votemmo getting listed separately. No clue why they're not grouped with Spieler.

2.0.1-b206 @ Spigot 1.8.8
I know, a bit outdated. Planning to update soon. Anyway I took a look at the earlier issue reports and could not find anything similar, so there might have been no change in this code since then.

Updated to 296, but no change.

Closing due to inactivity; I've also never encountered this. If this is still a problem, please open a new issue.