Otherwise panels can overlap which sucks
I've investigated this bug, it's another case where QScreen seems to be the cause. I have two screens here (extended desktop, no cloning), adding a panel does not seem to update QScreen::availableGeometry() and this then still returns the full screen size without struts by the other panels. However it seems to work for primary screen (only) when the panels are loaded on plasma startup. This may also be a bug in our panels setting struts properly (I found one already) or QScreen reacting to the struts (or NET::WorkArea rather). Furthermore, the panel sizing & positioning methods actually do not use the availableSize/availableGeometry methods, but always the "full screen" size/geometry methods. All in all, QScreen seems to be the culprit in this bug.
Is #322480 a dup or just related?
Related, yes, duplicate, not so much. Your comment on that bug (comment 11) is I think a different problem - when you place a panel at the shared screen edge, that panel is ignored by kwin and that's deliberate - see https://bugs.kde.org/show_bug.cgi?id=167852 This bug is about panels ignoring other panels (all of them) when changing that panel size (automatically or manually).
*** This bug has been marked as a duplicate of bug 336374 ***