See discussion below. Extended-confirmed and above (e.g., sysop) accounts have permission to edit semi-protected pages on the desktop site, but this is apparently not allowed on mobile. Pinging the Trust and Safety folks to see what the reasoning is/was, and whether this is something we should regularise.
Video of original issue: https://drive.google.com/file/d/1-IaVIGn-TVk6zXN6lU82eTH0TBRQ3_oJ/view?usp=drivesdk
From OTRS: https://ticket.wikimedia.org/otrs/index.pl?Action=AgentTicketZoom;TicketID=11088016