Consider you are trying to show a column in an incident related list but it isn't displaying.
For every incident, there will be a related list with Child Incident that shows records if there are any child incidents associated with it.
The related list shows as:
Now say if you want to modify the list as below and add the Parent Incident column.
The column will not reflect once you save the List Layout with the new column.
Since you are already viewing the parent incident record, and in the related list, filtering by Parent Incident = record, it doesn't show that column.
This is an expected behavior of the platform.