I agree. There may be many different and good ways to articulate the same thought/idea in English, but they may not all be in the Owl's computer data base. It is best to go with what you know she wants and not risk losing a heart and maybe having to start over just to see if a different and valid translation is in the data base.
It's an old method of motivating users. Duo no longer (as far as I know) uses that approach. My understanding is that one started a lesson with a number of hearts. When a mistake was made, a heart was "lost." If you lost all of your hearts, you had to start the lesson over. Losing hearts was serious business, unless you had a high tolerance for frustration.
In English research is usually "investigaciones" (plural) and it would be "I am doing research." for "Estoy haciendo investigaciones."
"una investigación" could be translated as "a piece of research", but in English we just don't say "I am doing a piece of research." We would use the plural above for research. You could say "I research into.....", but there is nothing being described as being researched. The best translation is already given: "I am doing an investigation." http://dictionary.reverso.net/english-spanish/research