I have noticed this a few times now. Comet C/2018 Y1 (Iwamoto) will disappear, sometimes on the same day even after I've updated minor body orbit data. Not only is it missing from view, it doesn't come up in a search. This is with Sky Safari 6 Pro. With other comets I have to manually update minor body orbit data to keep their position accurate but they don't disappear and always come up in a search. Comet C/2018 Y1 (Iwamoto) seems to get removed from the database when restarting the app for some reason. Btw, I checked SS 5 pro and it has the same issue.
Why Does Comet C/2018 Y1 (Iwamoto) Keep Disappearing? (Answer: Updated Orbit Data Is Stored In The "Caches" Directory - Free Up Some Storage Space)