SUMMARY
When duplicating a sequence, edits to the duplicate are not saved until the project is saved and reloaded once. 

STEPS TO REPRODUCE
1. Duplicate a sequence.
2. Edit the duplicate in any way.
3. Save the project.
4. Close and reopen the project in kdenlive.

OBSERVED RESULT
The edits made in step 2 have NOT been saved, it is still identical to the original sequence.

EXPECTED RESULT
The edits made in step 2 should have been saved.

SOFTWARE/OS VERSIONS
Kdenlive: 23.04.3 (flatpak)
Linux: Ubuntu 22.04.3 LTS
KDE Frameworks Version: 5.108.0
Qt Version: 5.15.10
MLT: 7.15.0

ADDITIONAL INFORMATION
Some additional observations:
a) Edits made to the original clip in step 2 are saved as normal, and do not affect the duplicate.
b) After creating the duplicate in step 1, the user is not allowed to save, perhaps indicating that kdenlive deems the project unchanged.
c) The bug seems to affect timeline changes of the duplicate sequence. Changing the name of the sequence gets saved normally.
d) As a workaround, I can make the duplicate; do another minor edit to the project (like changing the sequence's name) so I can save, see the items above; close and reopen the project; from this point onwards, all edits to the duplicate are saved as normal.

This bug has cost me several hours of work (despite saving regularly and even having backups), so I marked it MAJOR.