Any of the layers could had gone white. First or last. Of course, if probability is fixed for all them, the more layers, the bigger the probability of getting a clouded mini.
Just like with sprays you have to check weather conditions and distance, with brush you have to check dilution and avoid going over half dry half wet zones. Some varnishes seem to go worse with thick layers, like if they never cured if too thick.
Some cases of frosting can be solved by applying a new gloss coat. But if the problem is "deep inside" the varnish, no idea. Get a varnish that goes fully transparent, no matter how thick. Also getting a removable varnish could be a good idea, if something goes wrong, remove and try again.
I decided to run an experiment and just put a couple of big drops (Vallejo Matte and Gloss) over a plastic, and I will check how transparent they become. First impression (I always forget about it, but get reminded as soon as I put some in a palette) matte has some impurities and needs lots of stirring (and water) to become fluid and homogeneous. Probably the matting agent formed clumps.

Edit: the results are in. They both crumpled/spread, ending thinner than the original blob. Bubbles are nasty in both. The gloss one has a slight white/blue tint in the centre but otherwise rather transparent, and the matte one ended being translucent white, with nasty particles in some places.
Another quick experiment: undiluted matte but spread thinner gave a whiteish result, weaker but not nice. So I will continue using them thinned down with lots water, making sure there are no bubbles or particles before applying them in very thin layers (and only as last resource, when spraying is not an option).
Next: get other brands/types, specially removable ones, and best if they are near transparent when in the pot, but all acrylic ones I have seen seem to be milky.