sampleImage, vector layers and continuous rasterization

When I use the sampleImage expression on an imported Illustrator layer, I get an incorrect (0,0,0,0) result. If I convert that layer to a Shape Layer, OR if I simply turn on Continuous Rasterization, it works correctly.  What is really weird is that if I leave the Illustrator layer with Continuous Rasterisation OFF, pre-compose it, and then enable Continuous Rasterization for the Precomp (not for the original layer), I also then get the correct result.

Is there a logic to this that I am not seeing? Or a bug?

Is there a workaround I can use so that it works correctly with simple imported AI layers that remain non-continuously-rasterized?

I'm using AE 16.1.0 on Mac. Thanks