It's possible that the printing process used, perhaps screen printing, would be susceptible to solvent-based varnishes.
You already have the Tamiya acrylic gloss varnish - use this mixed with Tamiya Flat Base (not Flat Clear) to obtain an acrylic matt varnish. You'll need to add a small amount toe the gloss, mixed in a suitable small container, and then test it on something to see how it dries. If it shows a chalky appearance, or grey streaks, then too much FB has been used - add more gloss. If it dries gloss, or semi-gloss, then add a touch more FB, again testing, until the correct mix is achieved.
Alternatively, you could use Tamiya Flat Clear, or any other similar acrylic, just for these small areas, which could even be brushed on, using your normal, enamel varnish for the rest of the model.