Here’s an old thread about this, My Zwift only uploading map, and not screenshot to Strava
but more importantly, this reddit thread says it’s related to screen shots taken with F10:
https://www.reddit.com/r/Zwift/comments/jvbams/zwift_not_uploading_screenshot_images_to_zwift/
I exclusively use F10, and indeed, screen shots are not uploading to Zwift’s feed (The at Home Cycling & Running Virtual Training App) nor are they uploading to Strava.