hook_form_alter() not being called

Problem/Motivation

hook_form_alter() is not being called when a new item is added to a paragraphs field on a node.

Steps to reproduce

- have a content type with a paragraphs field
- use hook_form_alter() to set a non-paragraphs field to "#disabled => TRUE" (a checkbox, in my case)
- click "add" on a paragraphs field (no need to add any content)
- click "save" on node
- hook_form_alter() is NOT called (meaning the empty value of the disabled checkbox field is saved)

Note: I can edit any other field on that form and the hook_form_alter() function is called on node save (so the disabled state is passed through and the disabled field value stays as submitted)