Hello, welcome to the Community, and thanks for reaching out. This is indeed strange and not normal behavior for the back button. First, check in Settings > System Gestures > System navigation, and make sure the back button is not set to open Gallery.
If it is set normally, I would next make sure to uninstall any third-party applications recently installed, especially if it coincides with when this behavior started. Sometimes these apps can change settings in your phone.
After that, try clearing your device's cache partition with these steps: - Power off your device. - While the device is powered off, simultaneously press and hold the power and volume up buttons. - Once the Samsung logo appears, release both buttons. Your device should boot into recovery mode.
Using the volume buttons to navigate and the power button to select:
- Navigate to "wipe cache partition" and press power. - Select yes to confirm. - Navigate down to "Repair apps" and press power.
Be sure to also check you do not have any Routines set for the back button.
