I'm not able to reproduce this, regardless of login state (anon or logged in). :-/

Furthermore according to codesearch there aren't too many matches for "mw-diff-movedpara"; the ones relevant to WMF setup are MW core and the MinervaNeue skin. MinervaNeue merely hides the pseudoelement.

Is the bug still present there for you? Are you sure there's no site/user/gadget-specific CSS which could be causing this?

This isn't a bug but instead two different operating systems and/or font stacks selecting a different appearance: both appearances are valid. I recall the Unicode Sol being used liberally on en.wp as a marker for the sun and some OS update changed the appearance to something similar on the left for some people, which got a VPT thread. It was long ago enough that I couldn't tell you whether it was a new font stack or OS.

I don't think there's anything to be done here, but you could sell me on this being a task to reduce the available font sets in an attempt to make it consistent. Either way, not a bug.

Sure that this is not a self/site defined CSS rule (tested in safemode).