Here's a response.
I work at Mullvad. (co-CEO, co-founder)
Some aspects of the described behavior are as we intended and some are not. The cause is not exactly as described in the blog post. As for mitigation, we are already testing a patch of the unintended behavior on a subset of our infrastructure. If any of you try to reproduce the blog post's findings you may get confusing results throughout the day.
We will also re-evaluate whether the intended behaviors are acceptable or not. Some of this is a trade-off between multiple aspects of privacy, and multiple aspects of user experience.
Please note that this is my current understanding, which may change. I was only made aware of this an hour ago, and most of that time was spent talking with Ops, considering what to do immediately, and writing this post.
Finally, for those of you who do security research: when you find a security or privacy issue, please consider notifying the maintainer/vendor before publishing your findings, even if you intend to publish right away.