Sorry, I was thinking file browser mounts would appear in mount, but they don't.

You should be able to list file browser mounts in a terminal using gio mount -li after mounting via the file browser, and it will list the SMB mount it's using, ie smb://SERVER/$share/

This annoyingly doesn't give us the username or domain for the SMB share, and to get that if the server and share looks OK we have to run gvfs (what the file browser for PopOS uses in the background) in debug mode and re-mount the SMB share; in a terminal run pkill gvfs; pkill nautilus; LANG=C GVFS_DEBUG=1 $(find /usr/lib* -name gvfsd 2>/dev/null) --replace 2>&1 ; this will unmount anything in the file browser but will show what username and domain the file browser is using to access the SMB share, for example after clicking on a share in the file browser, among other logs, I get;

smb: do_mount - URI = smb://absolution.local/samshared
smb: do_mount - try #0 
smb: auth_callback - kerberos pass
smb: auth_callback - out: last_user = 'samblack', last_domain = 'SAMBA'
smb: do_mount - [smb://absolution.local/samshared; 0] res = -1, cancelled = 0, errno = [22] 'Invalid argument' 
smb: do_mount - enabling NTLMSSP fallback
smb: do_mount - try #1 
smb: auth_callback - ccache pass
smb: auth_callback - out: last_user = 'samblack', last_domain = 'SAMBA'
smb: do_mount - [smb://absolution.local/samshared; 1] res = -1, cancelled = 0, errno = [22] 'Invalid argument' 
smb: do_mount - try #2 
smb: auth_callback - normal pass
smb: auth_callback - reusing keyring credentials: user = 'samblack', domain = 'ABSOLUTION'
smb: auth_callback - out: last_user = 'samblack', last_domain = 'ABSOLUTION'
smb: do_mount - [smb://absolution.local/samshared; 2] res = 0, cancelled = 0, errno = [0] 'Success' 
smb: do_mount - login successful
smb: send_reply(0x55ea6ffe5450), failed=0 ()

This should give the username and domain that connects and can be used in the credential file.

Once this is done, you can exit the terminal with gvfs running and you should be able to close and re-open the file browser and the mounts should just re-appear normally.

Hopefully this will give enough information as to why the file browser mount works and the mount command doesn't.