Alex,
I use MPL files to import meta information for tagging artwork images. I have scripts that transform tag information stored in Excel into MPL files. In MC11.1 I simply moved the images files to the corresponding folder, selected the mpl file and right-clicked on import in Windows Explorer. Or imported the Mpl file directly from inside MC through import function. There was no need to drag the Mpl file to Playing Now, never did that.
This does not work any longer in MC12. I tried in different ways, just one or two files, newly created libraries, my standard library, with Otto on and off. I had to backup to MC11.1, but this is definitely not a long time solution.
In any case I will try your test file on Sunday as I will be off-line tomorrow.