Acrobat Reader for Android does not display ligatures such as "fi" in Reading Mode

Hi, I'm using Adobe Acrobat Reader for Android version 17.2.173338 an LG G4 running Android 6.0, also on a 2013 Moto X running Android 5.1, and 17.2.173339 on an x86 Lenovo Yoga Tab 3 Pro running Android 6.0.1.

When reading technical papers in the traditional two-column layout I always switch to Reading Mode to avoid having to navigate back up and side-to-side. When I do so I notice in documents using traditional Times Roman-like serif fonts that "fi" simply doesn't appear: "specification" appears as "specication", "sufficient" as "sufcient". Similarly "fl" doesn't appear: "reflect" appears as "reect". For a sample PDF, search for "laumann-munksgaard session types for rust", it has all these words.

Why does this only happen in Reading Mode? Can I fix the problem if I install "better" fonts? Please fix this bug. !