… way too lazy to re-enact your workflow, Jonathan (for days >32°C here…)), but by description, with all your manipulations of it you 'transform' a layered still into an animated clip, esp. a compound clip - which, Ben explained, behaves differently, when altered in timeline (read in the Manual about Independent and Parent CCs)
help.apple.com/finalcutpro/mac/10.4.1/#/verca9e8d33
… guess, you could optimize your workflow, screwing stills, clips, compounds here&there (own timeline, storyline, browser, project) …