This is expected. Startx2 queues a pattern to launch on the next checkpoint, so when you press start for the second time, you're queuing up the pattern to play again before it moves on to the next.
Quote:While playing, pressing start on another selected pattern will trigger the pattern to play after the last pattern of the same column finished it’s sequence. Pressing START 2 times, will trigger the next pattern faster, after an amount of steps, set up by the Project setting ‘Live Quantize’, derived from the main 64 step clock. You can see the next point of the starting point on the top and bottom Base Clock Indicator.