I can't say for sure, but waaay back in the day, I used to brush Tamiya acrylics with no troubles. Sometime around the early 90s, that changed and today I refuse to brush Tamiya acrylics.
They spray very well, but when I brush them, even thinned, they tend to clump up. I experimented awhile ago trying to find a workaround - adding gel medium, adding more thinner, more pigment (by scooping it from an unshaken second jar), but had no luck.
I think it's the carrier itself, it seems to be an aggressive acrylic that skins too quickly when brushed. It 's aggressive because it immediately attacks any coats that are not completely dried underneath.
I love the way it sprays, but when it comes to brush painting, I'm stuck on Vallejo for acrylics and Humbrols on the enamel side.
Jeff