We've given it another try.
I assume you have debug mode enabled (rather than development mode). Development mode enables debug mode implicitly but there's a few other differences. One of them being that the CSS cache is enabled in debug mode but not in development mode.
As a result, if I turn development mode off but I enable debug mode, and I try the test again, indeed I can reproduce the issue. So, this does seem to be a bug Which is a relief, I'm sure (for you at least! ).
I assume you have debug mode enabled (rather than development mode). Development mode enables debug mode implicitly but there's a few other differences. One of them being that the CSS cache is enabled in debug mode but not in development mode.
As a result, if I turn development mode off but I enable debug mode, and I try the test again, indeed I can reproduce the issue. So, this does seem to be a bug Which is a relief, I'm sure (for you at least! ).