Welcome Ksheetiz Sharma,
Thank you for the detailed explanation and for listing the troubleshooting you have already completed. Since the clock and widgets are still visible when this happens, while only the background image is black, I suggest testing the startup behavior and graphics driver rather than repeating the repairs you have already tried.
- Temporarily disable Fast Startup
- Press Windows + R.
- Type control and press Enter.
- Open Hardware and Sound > Power Options
- Select Choose what the power buttons do on the left
- Select Change settings that are currently unavailable
- Under Shutdown settings, clear Turn on fast startup (recommended)
- Select Save changes
- Shut down the computer completely and turn it back on
- Repeat the shutdown/startup test several times, since the issue does not occur on every boot
- Reset the graphics driver when the problem appears
When the lock screen appears with the black background:
- Stay on the affected lock screen.
- Press Windows + Ctrl + Shift + B once.
- The display may briefly flicker or you may hear a beep.
- Check whether the lock-screen background appears afterward.
Looking forward to your update.