Edited to add: With help from Sitecore Support, there’s an explanation for what was going on here now.
Edited further to add: According to the release notes, this issue is resolved in Sitecore 8.2 update 6.
If you spend your life working with software, you can’t help but collect a few stories of issues that defied your understanding – and I came across a great example with Sitecore recently. I haven’t managed to decide if this is an issue that can happen to others, or whether it was completely specific to the setup of this particular site. But since I got few useful results from Google when I was trying to solve this, I figure it’s worth writing about it just in case someone else sees a similar problem in the future… Continue reading