This is likely two different issues.
We do have as a known issue on Android that, if the app is closed, it doesn't retain the user's login information on being reopened. This is an issue specific to Android, and hasn't been observed on other platforms.
We haven't seen an issue on being signed out after an amount of time has elapsed. I'm currently testing this on both Android and iOS.
Can you confirm if one or both of these behaviors have appeared on both Android and iOS?