Yeah, it's a property of Tamiya paint when brushing over another layer of Tamiya paint. You can pick up another bottle of flat black paint from Testors Acryl or Polly Scale line of acrylic paints. They don't have this problem with brushing over another layer of paint that's not fully cured. Polly Scale Grimy Black is an excellent color for tires.
The one thing with Tamiya acrylic paint, and Gunze to a lesser extant, is that it is very tricky to hand brush.