@Jay Did you include any fixes for this bug in the recent Mirrors update?
I ask because my Agenda Pane has recently changed behavior (I only have one in regular use). I now have headings within the Pane that show a count for how many items should be inside them – but NO items are displayed. I can change filter settings and the headings will appear/disappear and update their counts to reflect what ought to be showing, but no items are displayed.
My Mailbird instance (stable version, obv.) displays this Panes correctly, with identical headings and counts but also showing the items under the headings. I created a new Agenda Pane in another Board/Document and it shows some items but others are missing. I haven’t figured out the common thread between the items that are shown vs. missing – but none of them are mirrors.