Normally the Android popup for an accent key choice appears above the keyboard row. But sometimes, from the top row, it goes below the whole keyboard and is very hard to access there. Nexus 7, landscape, sys 6.0.1. I am using several Smart Keyboards for Duolingo. This is a bad error and really annoying.
Same problem here (Pixel 2). While waiting for a fix, I found that enabling number row helps.