Sorry I don’t know of any better work-arounds, but speaking from experience (of being here on Memrise for what, 6 years now?), and based on the fact that this has been a problem with both the app and the real site (Chinese font used for Japanese etc) since the beginning, I really don’t think Memrise staff will fix it. It only takes a few seconds to switch your phone font before/after you use the Memrise app, though I personally just always leave it on since I’m almost blind and need the extra enlargement everywhere anyway.
I also use this kind of magnifying sheet that snaps into the headphone jack of my phone, so on top of enlarging the font I also enlarge the entire phone screen.
If you do come up with a better solution, I’d like to try it too!