I seem to be having the same problem Geo.
I used to mix my own matt or semi-matt clear coats, using a mix of 'Future' and Tamiya Flat Base, the amount of FB determining the degree of matt, from full matt, via semi-matt to semi-gloss.
You could try this, which has worked exceptionally well for me for years, but the last time I tried it (on the P-47), I got a few areas where it 'speckled' and/or went chalky. Now this might have been the 'Future' which, being the 'newer' stuff, already a milky color as opposed to the original, crystal clear product, was a couple of years old, and had started to gain a slight brownish tinge.
Try it by adding the Flat Base a bit at a time, stirring thoroughly, and coating a piece of scrap and letting it dry thoroughly. Adjust the amount of FB until to you achieve what's required.
I've since tried various matt acrylics, and they've all dried 'chalky', even when used and thinned as directed, and that includes the normally excellent Xtracrylic matt from Hannant's.
Maybe it's because of changes in formulation due to new 'rules', who knows?
But I'm still looking for a good matt varnish, and to an extent, a replacement for 'Future'.