@JimmXinu -- As I'm probably the only person who uses it, I think I have idea for an alternative method that doesn't use custom_columns_settings.
EDIT: Alternative method didn't work (field failed to populate)

Is it because I didn't define any default value for 'subtitle'?
Code:
[defaults]
custom_columns_settings:
## Subtitle
subtitle=>#subtitle
# Custom metadata entries
add_to_extra_valid_entries:,fulltitle,subtitle
[https://archiveofourown.org/works/43792521]
add_to_replace_metadata:
title=>^.*=>The Zhorusian Saga
subtitle=>^.*=>Rise of The Terra-Rannochian Commonwealth