I now have autofs working again.
With this new version of autofs/[osmc mount] the username cannot contain spaces or any form of escape sequence using ‘\’ and octal/hex character codes. This applies both to its explicit use in ‘username=’ of the ‘/mnt command’, and also when used in a .smbcredentials file.
I am now using a username without spaces (a microsoft account name infact) specified in a .smbcredentials file, which is infact more secure (because it is no longer part of the auto.smb.shares file (which is backed-up), and can also be seen in the debug logging.