Windows buffer_save_async() adding a 'default\' prefix to filename argument.

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I've run into a strange problem while using the function > buffer_save_async().
Occasionally this function will apply 'default\' to the start of the filename argument.
For instance, saving a BUFFER which contains a png file:

FILE = buffer_save_async(BUFFER, "bin\img\image1.png", 0, BUFFER_SIZE);

will save the BUFFER successfully, but to "default\bin\img\image1.png" instead of "bin\img\image1.png".

This seem to only happen occasionally, so it has been quite difficult to replicate on demand and debug.

Does anyone know under what conditions this 'default/' prefix is applied?
Hopefully i'm just overlooking something simple.

(GM:S-v1.4.1763, wYYC)