Hi
When you create a new custom field it has (1) name & (2) Display name
When you go to form editor & add this field to the form, and double click on the field to see the field properties, you will see (3) Label & it is automatically checked for you to show it on the form.
If you are making change in (2) Display name of the field ==> you will not see this in the entity record form, rather it will be seen greyed inside/within the field on the form editor.
If you are double clicking on the field in the form editor and updating the (3) Label text ==> then you will be able to see this text reflecting on the entity record form.
In all cases do not forget to Publish All Customizations.
Let me know if this is clear and helped you.
Thank you,
RBaliga