This function is documented as “The system will take its own internal copy of the data, so you can free the block once this method has returned”. And that was indeed the case.
Now, however, on macOS and JUCE 4.3.0 it is no longer true. If I get rid of the original data, the font I create from the typeface doesn’t work. If I refrain from deleting the original data, the font created is fine.
I don’t see exactly where this problem is originating, though.