>>We must not make something up that could misrepresent the data.
If it's missing from the cube it should be set to missing in the grib message.

Having looked into the issue, and scanned available files, I don't believe it is this clear-cut.
I have not seen 'missing' leveltype in any data we have, where you might expect to.
We are consulting with ECMWF for additional guidance on this.