If Windows explorers displays a space character in front of the filename, then there might be a character, which looks like a space character.
Basically, the Windows explorer does not allow to manually create a filename with a leading space character, but in a commandline window or by Mp3tag automatism it is possible to create such a filename.
If Mp3tag has created this filename, then the originator of the space character is almost certainly you.
You need to investigate vigorously the scripting expression and find out, how the leading space character has been introduced.
If you will be not successful with the investigation, then a foolproof method will be:
surround the complete scripting expression with a function $trim().