"Yesterday was June sixth."
Yep just typed 昨日 and told me I was wrong lol. A lot of kanji you type is told to be wrong, kinda tough for testing out of levels
It's possible that older versions of the app had an incorrect pronunciation.
You dont actually need the particle wa (は) when you use words for today, yesterday, etc.
In actual speech in Japan, you don't need the "は" after time expressions like "きのう".