Filter doesn't consistently return comment bodies

Question

I've been trying to set up a filter I can use with /questions/{id}/answers/ (using the UI at this link), which returns the following fields:

answer:
    answer_id
    body_markdown
    comments
    creation_date
    owner
    score
    share_link

comment:
    body_markdown
    link
    owner

shallow_user:
    display_name
    user_id

The problem is body_markdown. No matter what I do, the filters don't return it. Sometimes they do, if I switch off some of the other fields, but even then, if I actually want to use the filter in a request, it breaks again.

For example, I created the filter: !)ayn4w2btvFUpB1d6.o.BKKcUINGse1wa_ZUcP23R8RvI .
It should contain all of the above fields and, if I query it at /filters/{filter} it reports to include all the fields I want.

But if I actually use it, either testing it in the docs or sending an actual query the comment.body_markdown field is not present.

What's going on here?