I suspect that the OP meant how messages appear in the message-list view (preview lines), not in the individual message view.
I know Kostya made some changes in the recent version of Aquamail, but at least previously, Aquamail has been analyzing only the "plain text" portion, and possibly only some limited top portion of the message for the text to show in the preview lines in the message view. (So an empty or absent "plain-text" portion was resulting in an empty preview.)
I believe, Kostya has made Aquamail looking at the html part under some conditions (when plain-text portion was missing? or ..?) I suspect the range that is analyzed might still be limited. And I suspect that some HTML-formatted messages with inline images at the top might still "circumvent" that analysis, yielding an empty preview.