The problem that the bilibili video needs to be reset the next time it is opened after it is full may be caused by the cache problem set by the application. You can try the following actions to solve this problem:

1. Close the application in bilibili to ensure that the application exits completely;

2. Open the phone settings, find the application manager, click the "beep beep" application, and then click "Save";

3. On the storage page, select "Clear Cache" or "Clear Data";

4. Restart bilibili's application, open a video and reset the screen size;

5. After setting up, try to close the application and then open it again to see if the problem is solved.

If the problem still exists, I suggest you check whether there are any settings about screen ratio or screen size in the mobile phone system settings. Adjusting them may have an impact on the video filling effect in bilibili. I hope these suggestions can help you.