The official announcement has been posted. Yes, Midyear Mayhem is back for a second time this year. Here is a summarized mobile-friendly version of what you need to know.
Today’s June Crown Store Showcase included multiple references to midyear mayhem. This confirms that Midyear Mayhem returns this month.
I did a quick check of my ESO calendar and this might be the first time Midyear Mayhem occurs during the actual Midyear month (Jul 2017, Jul/Aug 2018, Jan 2019, Jan 2020)
The official word: Stay tuned™.