Render > selected object with alpha in front of refractive object disregards alpha

[Pixelcon]

Tricky title, but this object is in front of a glass balustrade and as you can see, the alpha is not calculated.
Not sure if this is intended or a known issue.

Not really looking for mask-based workaround suggestions, just raising the issue...

[hermit.crab]

Is the balustrade shader's refraction affecting all channels? Maybe attach the scene (simplified) so we can take a look.

[Pixelcon]

Im finding it really hard to pin down consistently to be honest. I had thought this morning it was only an IPR thing, but the render I attached above proves this untrue.

[hermit.crab]

Best send the scene so we can take a look.

[Pixelcon]

Here's a scene.

Very confusing. If I ensure affect all channels set for both reflection and refraction of the glass, it's fine, but if this is no the case for reflection, weirdness ensues.
Also the non-black alpha parts seem to render additively each time I re-render (frame not clearing between each render). This is probably 'normal' but it's the reflections that are disregarding the alpha? I'm not sure...

[hermit.crab]

Here's a scene.

Very confusing. If I ensure affect all channels set for both reflection and refraction of the glass, it's fine, but if this is no the case for reflection, weirdness ensues.

Yes, you should keep both refl/refr as "Affect all channels".

Also the non-black alpha parts seem to render additively each time I re-render (frame not clearing between each render).

You can change this behavior through the "Previous render" option (Settings>System>Previous Render). "Clear" will clear the frame between renders.