Using mp3splt-gtk-0.9.2 with libmp3splt-0.9.2 incl. id3tag support, splitting an mp3 in one part (i.e. using two split-points) would not result in a file with the expected id3tags if preferences->tags->"split file tags" is set to "original file tags" or "extract tags from filename". Both cases would result in the same tags you get with "Custom tags (from the splitpoints table)", that is, empty tags by default if no custom tags were set in the splitpoints table.
How fixed (for developers): In every case, a tags_group is appended to the mp3split_state for every splitpoint, with the custom tags data. Then at a later time, in case of "original file tags" or "extract tags from filename", an additional tags_group is added with the expected tags. But since only the first splitpoint's tags are written to file, the last group is ignored. So I just added an if-clause which only appends the custom tags_groups if the "custom tags" mode is actually set.