My guess is can was too far away, paint dried before it got to the surface.
I’ve done this a few times with various flat clear coats using an airbrush.
Years ago Stikpusher taught me a neat trick. Lay down a wet coat of gloss over the frosting, it should clear it up and when dry you can try again with the flat. Stik’s trick has saved me more than once. Seems counterintuitive to lay down a gloss coat over flat, but it works.
I’m not one to bet on the results of lacquer on top of enamel because I’ve seen it work and I’ve seen it go gittiwonkers too. Usually it’s not hazing though but rather crazing or wrinkling and it may not be right away either, though it could be… Wrinkling could be pretty immediate, crazing could be in a year or two or more. Not saying it will, just saying I’ve seen it and more than once.
Proper mixing is VERY important, not just a quick shake or a stir but thorough mixing at least a minute or two.
I spray flat clear lacquer over enamels and acrylics all the time with no issues . The first couple of coats need to be light so they dry fast reducing any type of reaction on the acrylic or enamel. I let the acrylic and enamel cure for at least a couple of days before spraying over the lacquer dull coat.
You may have applied the MM flat laquer too heavy. It comes out much faster than paint in a rattle can. Light mist coats are recommended.
There is no problem whatsoever applying this over enamels. I have done that with over 200 models. MM enamel airbrushed then MM flat from the rattle can.
*You must let the enamel (or any paint for that matter) dry for at least 48 hours if not for several days. Then flat in mist coats then a slightly wet coat.
I have had Testors clearcoats blush. That is the white appearance due to high humidity. Lacquer thinners evaporate so fast they actually cool surface. If the dewpoint is too high- close to ambient temperature, it condenses on the paint and appears as a white fog. Usually this is summer time really damp weather. Indoor humidity in winter is usually ver low. It is the humidity where you are painting that counts. But in summer on warm days, try to watch out for really muggy or wet days and avoid lacquer glosscoats.