Just ran into this issue. No easy workaround either as I'm using a WFS layer with GML as the source, which only allows for an XYZ layout.

Assert errors are thrown in ol.geom.flat.deflate.coordinates(flatCoordinates, offset, coords, stride); as the original features have 3 coordinates (stride 3), and any new vertices only have an X,Y.

The goog.asserts.assert(coordinate.length == stride) therefore fails.

same problem here with GPX Layers (Layout "XYZM").
my workaround is:

ol.geom.flat.deflate.coordinates =
    function(flatCoordinates, offset, coordinates, stride) {
  var i, ii;
  for (i = 0, ii = coordinates.length; i < ii; ++i) {
    var coordinate = coordinates[i];

insert this:

    if (coordinate.length < stride){
      for (var x = coordinate.length; x < stride; x++){
        coordinate.push(0);
      }
    }
    if (coordinate.length > stride){
      for (var x = coordinate.length - stride; x <= coordinate.length; x++){
        coordinate.pop();
      }
    }

before:

    goog.asserts.assert(coordinate.length == stride);
    var j;
    for (j = 0; j < stride; ++j) {
      flatCoordinates[offset++] = coordinate[j];
    }
  }
  return offset;
};

See #3036 for a fix in ol.interaction.Modify.

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Please correct me if I'm wrong but as far as I can see there is still no way to specify geometry layout for ol.interaction.Draw?

I think you're right, @drnextgis

@ahocevar I've made a PR to fix that. Please review.