That’s a LOT of “missing panel”.
How many other instances of this particular panel being “missing: are there?
HOLY CRAP !!!!!
“Missing” implies fell off or not installed.
I would describe that as “mostly corroded away”.
That’ll create a little drag.
Neglect
They should have used aluminum instead of that cardboard carbon.
The damaged panel is made from kevlar face sheets with a honeycomb core, most aircraft fuselage and engine pylon fairing panels are. On Boeings they are not typically considered structural, as evidenced by the fastener spacing. Airbus panels are typically considered structural, with a much closer fastener spacing. This image shows the left side of the main gear wheel well, with the damaged panel just aft of the opening. The underside of the flap, about two feet outboard of the damaged panel, shows impact damage. Possibly FOD damage also impacted the damaged panel and started the failure of it. Fairing panels with a honeycomb core are also susceptible to water damage. A small hole can allow water to enter, which freezes and expands at altitude, causing more damage. The flap drive mechanism that the panel covers looks intact, it probably didn’t damage the panel from the inside. I don’t believe this is a Boeing problem at all. I would guess FOD from a rotating tire on gear retraction, or water ingestion and freezing.