Ah, I am now able to reproduce it.
It doesn’t appear to matter what the distribution list is named, but it has to do with the number of members in the list. If they cannot all be displayed in the second column, then the first column is reduced in width to try and compensate. But all you end up with is two columns that are both truncated.
If you have a Pro License you could open a support ticket with eM Client and report it as a bug. They may have done it intentionally though, but it does not look good. Rather leave the first column to auto-adjust (within reason) and truncate the second column.