Yes, when I said "opened" I followed the process you described. 
It might be worth checking the html code.
Open a RH7 topic in a text editor and check that you can find the drop-down content in the topic source code. (It might be right at the top of the topic, that is, separated from the drop-down hyperlink text. )
Open the same topic from a converted project and check that you can find the drop-down content. (In the same location as the drop-down hyperlink text.)
Thank you all for your help. Rick discovered that the project I was trying to upgrade had been created from webhelp output files which was surely the cause of the different issues I was having. I located an earlier version of the project and it upgraded to RH11 without issue. So although the content needs to be brought up to date, we finally have a clean, working project. Again, thanks everyone.