Hi Paulius,
This sounds like a bug we fixed recently and that will ship soon with version 101. The bug is that you will always get the prototype linked to the first artboard of the page you are viewing. The bug corrects itself when you switch artboards, but comes back when you switch to a different page.
However, if you’re experiencing something else, can you share with us a sample file to test with?
About the other two issues:
Sometimes the app would not let me to exit the prototype after holding on the screen
- Can you share a bit more details about this issue? For example: does it happen with a particular document? Or perhaps it happens with prototypes that have a particular layer structure
Mobile app and beta 101 files
It’s not possible to block beta files from the mobile app, but once the final version 101 is released, you can open the beta files with Sketch 101, make a change and save them.