The event handler for OnSpecGearTalentUpdate / Player Inspector triggers on PLAYER_EQUIPMENT_CHANGED and ZONE_CHANGED_NEW_AREA, which in turn calls Spell:ArtifactScan();

This ends up calling SocketInventoryItem(INVSLOT_MAINHAND) which hides the character panel automatically in the Blizzard UI.

Once these events are registered, the character panel will disappear every time the player swaps gear in the panel, or whenever changing zones.

I changed this locally to:

if Event == "PLAYER_SPECIALIZATION_CHANGED"
Spell:ArtifactScan();
end

But not really sure if that is the correct fix. It at least resolved the issue temporarily.