Just installed version 1.7.0.145 for Windows on a Win 7 desktop. Worked on a test project for about three hours off-and-on. Opened Help, searched for a topic and minimized. When I realized an hour later that I still had help open, I closed it and got the error that Help had stopped working -- check online for a solution or close program. I chose close and Help closed. After another few minutes, I closed the program using File > Exit and got the same error that Affinity Publisher Beta had stopped working. I chose close and the program window closed. Unfortunately, I didn't capture the information under the error message. If it happens again, I'll capture it.
I opened the project again and nothing was missing. I made a slight change, saved, and closed with no error.
This did happen once on a previous Beta build, and that project, too, had been on the screen for a few hours before being closed.