It’s hard to say from description without seeing visually (we are at techsupport at revisionfx.com). The removing of the solid should not affect the layer with RE:Flex.
Could be:
1) Do you have Smart Blend checked?
2) Do you have by mistake the viewer set to RGB Straight?
3) Could it be the Interpret Footage for Alpha Handling is Straight and should be Premult or the inverse?
4) What happens if you toggle your project between 16 bit and 32 bit float?
Pierre