Hi there, welcome to GrapheneOS and the discussion forum. Glad to hear that you're having a good experience with your new phone and OS.
It's not normal for the app to freeze and cause a soft reboot, especially if it occurs regularly.
Next time it happens, could you generate a bug report please? Immediately after the phone reboots go to Developer options and Generate a bug report there. We'll then see if a GrapheneOS developer can look at it, so we can get to the bottom of the issue.