@bwiernik you're right. Thanks for the explanation, I think in that case, if MDPI doesn't make it mandatory it is preferable to use the official CSL file or my modification (https://github.com/nelsoncardenas/MDPI-Alternative-Style-for-Zotero-Mendele…
If anyone needs it, I have made a csl file alternative with DOI detection. https://github.com/nelsoncardenas/MDPI-Alternative-Style-for-Zotero-Mendeley MDPI asked for the DOI some months ago, so I had to make it. Maybe, his policy has been updated.
As the Zotero file has not been modified so that MDPI adds the DOI (actually, it is very simple to do so), I have modified the style "multidisciplinary-digital-publishing-institute" (https://www.zotero.org/styles?q=id:multidisciplinary-digital-publi…