Grasshopper animation bugs involving transparency

I’ve noticed a few strange problems with creating animations from sliders in grasshopper when transparency is involved.

Here’s a minimal setup to reproduce one of the issues:

(Here’s that setup as a grasshopper file: (8.7 KB))

This makes a box that rotates as you move the Angle slider. If I choose Animate from that slider’s menu I see the right thing on screen, looking something like this:

However, the saved bmp files are… strange. In Preview they look like this:

If I open it in Photoshop I just get a blank white square.
If I open it in Sublime Text, I get this:

It looks like it is a bmp file with transparency? In any case, my other Mac software seems to be very confused, and the output doesn’t look like what I see in the Rhino window.

If the transparency slider is below 0.5 then the animations behave much better. I also noticed that the ambient occlusion effect on the ground plane only appears for transparency below 0.5. Here it is at 0.4:

I have also had other buggy, inconsistent behaviour from animations with transparency. For example, I’m rotating some object around, and on some frames the transparency appears correct, and on others the appearance changes drastically. I don’t have a way to reproduce this. Running the exact same animation multiple times can give inconsistent output.

Any thoughts or workarounds?

Finding my own workarounds: So far. setting the transparency below 0.5 and turning off shadows on the lights seems to be producing usable animations.