I tried to change Kuala Lumpur from a state to a city that’s in the state of Selangor but in the process, somehow whenever I do a search,
results from OpenStreetMap Nominatim shows “City Kuala Lumpur, Negeri Sembilan, Malaysia”.
I do not know why it’s now associated with Negeri Sembilan, I did put in the tag is_in:state Selangor. 
should have been is_in:state=Wilayah Persekutuan, not Selangor.
Nominatim probably went bonkers since we don’t have well defined state boundaries, even after so many years of OSM mapping.
Is it a better idea then, that it’s rolled back to what it was or should I put it as Wilayah Persekutuan?
If it is alright with you, I have reverted some key-value pairs to match the changeset by user katpatuka.
Before
After
Side note:
Katpatuka took some time to map the remaining state boundaries, if I’m not mistaken, early this year using public domain data. You can follow this changeset discussion to get the idea - I suggested following the tagging style of Commonwealth countries. But as you can see below:
Some tags perhaps were established long time ago.
Also… since Kuala Lumpur is the capital city of Malaysia, I reckon, following conventions, I made related changes as well.
Nominatim may take some time to update. Let me know if there are things that need to be discussed.
Thank you. 
Glad I could help.