Sketch 3.7 has been updated to save files in a better way such that missing fonts aren’t replaced with the system font until you edit the layer directly.
Sketch 3.7 and New Symbols

So this works even with one layer? For example, there are several layers which have missing fonts but a user (me) updates only one layer. Would rest of the layers stay intact and the replacement information is not saved or only the layer which has been modified would be updated rest will retain their font information after saving and closing. I hope I am making sense with my question :D

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