Beets scrub plugin ignore custom tag from scrubbing

According to description of scrub plugin, it only scrubs tags that are not tracked by beets

https://beets.readthedocs.io/en/v1.3.3/plugins/scrub.html

I have a custom tag "SUBBOX_ID" that I write to files. I want scrub to NOT remove this custom tag, but to remove everything else.

I've tried adding the following item_fields to my config but scrub still seems to remove the tag:

item_fields:

subbox_id: subbox_id
SUBBOX_ID: SUBBOX_ID

How can I get beets to track my custom tag so it is not scrubbed by the scrub plugin?