This is likely happening due the the board context of the automation looking at the parent board ID and not the subitem board ID where the subitems files column is.
So that we can look into this further would you be able to please fill this form adding as much information as possible to it (such as account ID, board IDs, item IDs, timestamps, etc.) so that our team can take a look into it?
Hi @dvdsmpsn. Shanine here from Mindflows! I understand that you want to do a custom action with Files. Would you mind sharing what type of custom action you want to achieve? This may be doable with Make/Zapier.