As this glitch is still not solved, I'm going to make a discussion about this.
My workaround is as follows:-
Instead of setting the date by clicking on the Due Date button, click the green Planyway button marked Set Dates.
If you want All Day check the box.
If not, uncheck it.
You can set the Priority level with this facility.
Then, to return to the Trello Card, click on button at bottom left and you can finish off with description and comments, plus labels and setting Watch.
This glitch only just occurred after the update of my Chrome browser to Chrome 79 (79.0.3945.79).
Before that everything was fine.