If you have ever searched for open source keyboards, you might have come across OpenBoard. But that project hasn’t seen development in more than an year. Thankfully, there is a fork of it that is being worked upon quite actively. It already supports autocorrect, glide typing, user-defined dictionaries, multilingual typing and now material you themeing as well.
Looks like some kinda scaling artifact. It’s probably from processing of the screenshot and not anything to do with the way the app actually renders fonts.