I wrote:
> I believe I understand what's going on here, and it's not quite as
> exciting as it first appears. The issue is that we are failing to
> honor the "toasting goes only one level deep" rule in the specific
> case of arrays of composite type. So while it's definitely a nasty
> bug, it affects only narrow use cases, and doesn't call into question
> our whole vacuuming strategy or anything like that.
I've committed a fix for this. Thanks for the report!
regards, tom lane