The solutions to the driven Van der Pol equation, and especially their derivative, change abruptly, so abruptly that if there’s not an integration point in the transition zone you get the long straight lines you see above.
With more resolution these lines go away.
This is understandable when you look at the plot of the solution: and of its derivative: The solution looks fairly regular but the varying amplitudes of the derivative highlight the irregular behavior.
The correct phase plot is not as crazy as the low-resolution version suggests, but we can see that the solution does not quickly settle into a limit cycle as it does in the undriven case.
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