This seems to only affect an emitter and not a system asset.
1. Create any new Niagara Emitter asset
2. Open the asset by double clicking on it
3. In the Niagara Emitter editor, in the timeline panel, set playback end to for example 2.0 seconds (under playback options button)
4. Save the asset changes and close the Niagara Emitter editor tab.
5. Immediately reopen the same asset
Result: Niagara Emitter forgets the changes to timeline settings and reverts to default duration of 10 seconds
Expected: Niagara Emitter editor remembers the playback range set up by user, instead of discarding them
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-99573 in the post.