Welcome, @anon35150446,
Right, only the top-level tasks and their immediate, first-level subtasks inherit the project’s custom fields.
But you can add the current project on the subtask (Tab+P in the right-side task detail pane or choose from Add to project from overflow menu there) thereby adding it also as a top-level task, to the project, thereby allowing it and any immediate subtasks of it to inherit the project’s custom fields.
You can optionally place it in a collapsed “Hidden Subtasks” section if you prefer.
Hope that helps,
Larry