There are several fixes or workarounds for this issue. On my systems, I had to remove the Windows credentials for the target computer using the Credential Manager. Then, I remapped the drives, typed in the username and password for the network share. That did the trick on my Windows 10 v2004 systems.

Reset the stored credentials using Credential Manager

Removing the stored username & password for the network share, and then remapping the network drive helps in some cases.

1. Open Control Panel → User Accounts → Credential Manager
2. Select Windows Credentials
3. Under Windows Credentials, click the chevron next to the name of the network computer (or NAS) that hosts the share.
   
4. Click Remove to delete the stored credentials of that particular computer or share.
5. When the Delete Windows Credential dialog appears asking, “Are you sure you want to permanently delete this Windows credential?”, click Yes to continue
6. Exit the Credential Manager window.
7. Open This PC and disconnect the mapped drive.
   
8. Restart Windows.
9. Map the network drive again, with the Reconnect at sign-in option enabled.
10. Restart Windows to check if the drive mapping works.

That should fix the problem. If not, follow the next procedure.

Add the “ProviderFlags” registry value

Adding the ProviderFlags (DWORD 32-bit) registry value and set its data to 1 helps some users, especially if your mapped network drive points to an old SMBv1 share. Follow these steps:

1. Right-click Start, click Run, type regedit, and then click OK.
2. Locate the following registry subkey:

   HKEY_CURRENT_USER\Network\[drive letter]

   Note: The [drive letter] placeholder represents the mapped drive.

3. Create a new DWORD (32-bit) value named ProviderFlags
4. Double-click ProviderFlags value. In the Value data box, type 1
   
5. Repeat the above procedure for each mapped network drive on the computer.
6. Click OK, and exit Registry Editor.
7. Restart Windows.

See if the mapped network drive persists across reboot.

Credits to user LeeB1430 for the above fix.

Map network drives using a batch file

If none of the above fixes help, then, as a workaround, you can map the network drive at every startup using a batch file.

1. Copy the following lines and paste them in Notepad. Change the network or computer name, the share name, and the drive letter accordingly to match yours.

   timeout /t 30
   net use z: "\\networkname\sharedfolder" /persistent:no

   The parameter persistent:no is equivalent to unchecking the Reconnect at sign-in option in the Map Network Drive dialog box.

2. Save the file as mapdrive.bat
3. Open the Startup folder of your user profile by typing shell:startup in the Run dialog.
   (To know more about the shell: commands, check out this article.)
4. Place the batch file mapdrive.bat or its shortcut in the Startup folder.

That’s it. The drive mappings are created at every startup rather than making it persistent across reboots.

Note: A similar workaround using PowerShell is available in the Microsoft Knowledgebase article Mapped network drive may fail to reconnect in Windows 10

Hope one of the fixes or workarounds helped you access your mapped network drives without any issues in Windows 10 2004.

Source from: 

https://www.winhelponline.com/blog/mapped-network-drives-fix-windows-10-v2004/