The following is a compilation from Slack of bugs related to Mirrors only. Even on their own, some of them are substantial -- but taken all together (and given how long they've been going on) I feel they constitute a complete failure of the "mirror" feature, and have thus tagged this "urgent".

They are confusing and workflow-wrecking for me -- and they can't possibly be working out well for other users, whether they know it or not. Several of these bugs directly lead to overlooked items if user is relying on filtered or group-by Panes to generate tasklists. @LauraH, @Jorge and I have posted most of these.

There's a fair amount of detail in slack already so I'm just linking to those Slack threads. One common thread is the idea that while all mirror-instances should be identical, in practice it's clear that one of them is in fact an "original". Regardless of how Legend handles these items "under the hood", the behavior the user sees needs to be consistent.

(March) Zooming on a mirror-instance takes the user to the original item, even if it's another document. Reported again here by Lala Rara. And again, by me.
(March) Creating a mirror by typing '((' creates an item that behaves like a LINK, not a mirror (even though the "delete" dialog box calls it a mirror). Probably related to this bug about Alt-dragging items created out of Mailbird.
~~(April) Dragging mirrors doesn't always put them where they are "dropped". Especially troublesome at the beginning/end of a series of children. Mentioned again here with screengrab~~.
(April) Group-By and Sort-By operations either ignore or completely omit mirrored instances. Another post here.
(April) [Filtered Panes also tend to ignore mirrored instances]. (https://moodo.slack.com/archives/C99CWQ484/p1618205888262000) Also here. And again here.
(April) Potential data loss when working with parents in multiple Panes.
(April) Overview Drawer ignores Mirrors
(April) Probably related to earlier bug, children of mirrored items that match filtered/group-by Panes are completely omitted if their parent is not the 'original' mirror..
(April) Mirrored items with a tag are not treated as if they have the tag (e.g. in filtered Panes). I believe this only applies to mirrors which aren't the 'original' but didn't verify when reading the post.
(May) Mirror instances that are Projects (Headers) display with the h1/h2/h3 formatting of the 'original' item regardless of the mirror-instance's position in the current document.
(May) MOBILE: MOVING a mirror creates a NEW item at the destination.
(May) The "Grandparent Zoom" behavior, which causes Mirror instances to not display in certain Outline Panes. Original by Laura, seconded here by me.
(May) Mirrors which have OneNote attachments won't open the OneNote document when clicked, if clicking on the non-original instance. Wouldn't be surprised if it happens with Evernote or other attachments, too (but didn't test).
(May) Dropping a Mirror onto the root-level of a document causes it to go into limbo -- disappears...but still exists.
~~(June) Alt-dragging to the end of a list of children in a different document creates the mirror at the location of the 'original' item instead of the target.~~
(July) GRANDParent-items that are mirrors are not treated as such in the Overview drawer (they aren't shown with a caret)
(July) Sometimes pressing <return> after a mirror-instance inserts the newline after the original instance instead (no reproduction steps).
(July) Sometimes alt-dragging an item creates a Link instead of a Mirror (no reproduction steps)