I'm seeing this behavior on tablets as well. Nexus 7 and iPad using Adobe Reader and Dropbox's default PDF previewer.
Using Scribus 1.4.4 Win7/64 bit build. I've tried exporting in 1.3/1.4/1.5 formats with no changes.
I've attached a very simple test case, literally a single form field on a blank page.
Work-around: editing the file in Adobe Reader IX, placing a single space in the text field(s), and saving the result seems to force correct display. Seems very much a kludge though.