ColorMask in second RT slot disables blending?

Hi.

I have a render-setup that writes to color target, but also a second render-target which I use to track certain material IDs. These are both bound during a pass and behave as expected under normal circumstances.

For a particular effect, I wanted to mask a channel of the ID target to prevent it being over-written. I used ColorMask in my shader to achieve this, as I've done before with non-MRT setups.

Unfortunately, it appears that using the MRT ColorMask syntax (i.e ColorMask RGBA 1) disables blending for the second render-target.

I confirmed this in RenderDoc by adding the ColorMask to my shader with all channels enabled for slot 1 (as in the above sample), which should theoretically be identical to not setting ColorMask at all.

The output merger showed the following:



Is this expected behaviour?