This always happens whenever I use the osu!mania editor, no matter if I'm creating a song's first difficulty or using an existing map and creating a new diff:
1. Fill out anything in the General tab.
2. Pick whatever settings in the Difficulty tab then set it to an osu!mania-only difficulty.
3. Place notes, even just one, then save. (If there's already notes coming from a previous difficulty you used, that's fine, too.)
4. Select Save as New Difficulty..., pick a name for the new difficulty, then change the Columns setting. (THIS IS IMPORTANT! Leaving the Columns setting alone won't reproduce this bug!)
5. Click OK then Yes. The notes from the previous difficulty should still be there.
I don't know if this was intentional, but if it is, the dialogue box should be changed. If it wasn't, then... yeah
1. Fill out anything in the General tab.
2. Pick whatever settings in the Difficulty tab then set it to an osu!mania-only difficulty.
3. Place notes, even just one, then save. (If there's already notes coming from a previous difficulty you used, that's fine, too.)
4. Select Save as New Difficulty..., pick a name for the new difficulty, then change the Columns setting. (THIS IS IMPORTANT! Leaving the Columns setting alone won't reproduce this bug!)
5. Click OK then Yes. The notes from the previous difficulty should still be there.
I don't know if this was intentional, but if it is, the dialogue box should be changed. If it wasn't, then... yeah