Found the fix. It appears the problem wasn’t directly related to the android app, but rather OH2 itself (version 2.5.9).
My nginx reverse proxy was using an older configuration. For whatever reason, when I upgraded from 2.5.6 to 2.5.9, the reverse proxy was no longer passing item updates to the rest api.
Adding the following 3 lines to my nginx configuration seems to have solved the problem:
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "Upgrade";