Is there a way to overwrite defaul font for a specific app Android 5.0?
From what you're saying, I pressume you're on a 'Nightly' or unstable version of a ROM, or if its an OTA update that took you to Lollipop, there's a bug somewhere. Keep calm and wait for fixes, or if you're impatient, then behold below:

By the way With the right search keywords, I found this called Fonter:


  Instead of only being able to change your entire system's font, Fonter allows you to choose separate fonts for individual apps. And with a huge library of free fonts to choose from, you're bound to find the right look for every app on your phone. - Quote from here


I have in no way tried the Fonter, but I am sure it might be the solution to your problem, for the time being, until the actual issue is fixed

However, since you say that issue of yours wasn't there initially, I will suggest you fix the underlying problem, rather than go around with a 'hack', kinda.