A distlist can get copied from a source and can only be updated in the source -  this is the only exception to bidirectional, even if the relationship is set that way

If a user gets a copy and adds users to it, their copy will stay the same and will only be reverted to the copy of the source if the source is updated, not on every sync

Sometimes a distribution group can appear "corrupted" to the sync program.  Normally a distribution group is made of Contacts from within the source folder.  If a distribution group has members in the group which no longer exist, the sync may have trouble copying the list.  Edit the group to remove any "strange" members or remake the group.   Distlist DistributionList