I'm sending this to the gdal-dev  on the assumption that gdal underlies QGIS's creation and export of shapefiles. Let me know if this is a bad assumption and I'll redirect to qgis-dev

Background - 
I created a vector layer in QGIS with layer type shapefile and geometry type linestring. I then used the drape processing algorithm to add elevation values from a DEM in the QGIS project. I then exported the draped layer as a shapefile. I then passed this shapefile to an online utility that converts the shapefile into a Litchi (drone app) compatible CSV file. The converter issues a number of warnings. I also used the Extract Vertices utility to create point geometry which I also exported as a shapefile. This, too, created warnings.

The author of the utility told me he is using a PHP library located here to handle the shapefile. The library author has this to say about the errors:
It is indeed what I was suspecting: QGIS is creating a malformed 3D (4D actually) file when there is no Z nor M dimension needed.
Basically they're missing binary data for Z and/or M dimensions either for the very last record (pretty usual) or sometimes ALL OF THEM (more rare, but I've seen some of them)!
He was not specific beyond that. 

If I load these shapefiles into QGIS and look at the geometry, the z-values are what I would expect.


The LineStringM and Point shapefiles are here.




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