Yes, thanks, we are perfectly aware. You can use both on Duolingo, both piji and piju are accepted.
This course teaches for the most part Standard Czech, not Common Czech or any of the dialects.
Piji is more formal and more used in writing, while piju is more colloquial and more used in speech. That is why piji is preferred here.
I absolutely agree! Language instructors need to realize the diacriticals really throw English speakers at first. - OK, the ů, ř, á and even ý are always clear and kinda cute - but those i / í are almost impossible to distinguish for us even as we continue (especially if we are 'older'). We need an i to have a diacritical almost like / !!