Would probably be preferable, since auto makes it behave like no z-index property has been specified (as is the case in the default Bootstrap theme used in lavender and persona) which makes the element inherit the z-index of its parent, whereas z-index: 0 forces the z-index to be, well, 0.
What I mean is -- we cannot fix issues if we cannot trigger them ourselves. We need a consistent set of reproduction steps (e.g. "go to this page, log in, press enter, navigate to this page, post.") in order to confirm that this is a bug and develop a fix. Otherwise if we cannot reproduce it, then it may be a configuration problem with your install, and we cannot fix that for you.