That's simply because if the WAV file would be longer, the (now unused) pattern 1 is used to stretch the module to the real module length. As you say yourself, it's really minor, and it's not even a bug, so I'm not going to bother with this.
I'm not even sure what the use case of importing such a short WAV as a module would be - you can load short WAV files directly into the sample editor without having to use this module import feature anyway.