Save Animation with transparency is broken

Save Animation with transparency is broken. Here is how to replicate:

5.12.0, Linux, builtin server
Load can.exo.
File/ Save Animation. Save as .avi. On the next screen, enable Transparent Background. OK.

The .avi will be bonkers. Here is an example using can.exo.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information