Adding any Expression Control to a precomp layer with Continuously Rasterize on breaks rendering

I have a precomp which contains a few layers in it, some with transfer modes like Multiply.

In my 'main' comp, the precomp layer is set to Continuously Rasterize, and the transfer modes of the layers inside the precomp are rendering correctly in the main comp.

However, if you add any Expression Control to the precomp layer, immediately the render changes as if all the transfer modes inside the precomp were set to Normal. It's still re-rasterizing the precomp after transforms, but the 'Quality and Sampling' switch changes from a - to / and the composited results are incorrect.

To duplicate:

1. Create a comp (we'll call it PreComp) and draw a red square; set it to Multiply and turn the opacity down a bit for fun

2. Create another comp (we'll call it MainComp) and draw a light blue shape over the whole canvas

3. Drag the PreComp layer in over the blue layer, set it to Continuously Rasterize, and note the visual result

4. Add a Slider Control to the PreComp layer, and note instantly the red square is no longer multiplied over the blue background correctly