digiKam isn't writing to Subject field for mp4s

Can anyone please help me figure out why the tags aren’t being written to the “Subject” field for mp4s? I’m thinking there must be a setting somewhere I’ve missed. It writes to the “Category” field but when I check with exiftool the “Subject” field hasn’t changed. This seems to be the field read by phone gallery apps that I’ve tried like F-Stop and Aves

Is there something I need to add in the Metadata tab of the Configure menu? Under the “Advanced” tab I can choose Tags from the drop-down menu and it looks like I can add fields? But I’m not sure what to write?

For background, I’m new to digiKam and am using it to organise my thousands of home video clips and pictures. I was previously using Synology Photo Station but was forced to “upgrade” to Synology Photos. I’ve been very happy to find digiKam which is much better, although it’s a learning curve for me. I’d be very grateful for any help offered