It's only a best guess, and you can untick it. GEDCOM file cards tick known dead for anyone born over 100 years ago (or whose parent, brother or sister was), so check it before you add them. On the merge page, if only the other record says known dead, you'll see ✓ Adding this – tap it so it says Add this, and they stay living. For example, say Grandad George is alive and well at 101 but his GEDCOM card says known dead – untick it and he stays with us.
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