I'll bet it doesn't even out.

The flats and mattes can be tricky to do. I know pros who have issues with them. (I'm no pro). Dealt with it last summer on a hood - not so much stripes as splotchy results in one area using a single stage black. What worked: slight over-reduction and/or a slower reducer and spraying the piece in two directions. If the piece doesn't look evenly wet all over when you think you're finished spraying, it may not dry evenly.