I've had this issue with 6 plus as well. After a period of normal functioning, when zooming in (using + & - buttons), the app zooms in to maximum zoom all by itself, and pressing the minus button does nothing. To get back to viewing the sky and zooming normally requires a reboot. Although the app will work normally initially, the issue is nearly always going to happen at some point during the night.
Like I said, I had this issue with 6+, and on both my current phone (Pixel 7A) and the previous one (Pixel 4A). There's a Cloudy Nights post saying the same wrt to other phones here: https://www.cloudynights.com/topic/758321-skysafari-pro-stuck-zoom-with-new-s21-ultra/
Very annoying indeed, so I hope you can suggest something.