not empty filter does not work with acknowledgements field in publications

[marton-balazs-kovacs]

According to the documentation, the emptiness filters also work on non-filterable fields. However, the following query returns 0 hits:

""" search publications where acknowledgements is not empty return publications[id+title+acknowledgements] """

In contrast, the query does work for abstract instead of acknowledgements.

Here is a link to the relevant page in the documentation:

Is this a bug? Or am I missing something? Thanks!