This issue is different for the first and second behaviors, which seems confusing when working.
1. Create two Level Sequences. (NewLevelsequence and subsequence)
2. Open one and add a Subscene track.
3. Add another Level Sequence to the Subscene Track.
4. Select and copy and paste. (A new Track will be created.)
5. You will also get a warning if you then select a new track and paste it.
6. Once again, select the first track and paste it. Then the paste positions will overlap and Paste.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-111983 in the post.