Regarding this source file generation (the new format introduced in Sketch 43)… Is there detail/setup to be aware of? Unzipping original.sketch and, then, just zipping it back produces a different file (compression-wise, at least) that Sketch is unable to open. I guess it relates to some compression setting but, as mattm said, documentation's lacking.
e.g. new-sketch-file.sketch weighs ~19ko when original.sketch weighs ~24ko.