When inserting a due date from the calendar it changes the text that shows up in the text box to the day before.
Click June 20 on the calendar and it will populate the text box with June 19. Then saving this makes the task due date June 19.
The 'today' and 'tomorrow' text addition links insert the correct date.
Mindstream v1.6.0 from both the Google Play store and sideloading from this repo.
Android Version 9