In Write Mode, I can actually copy a Note Spacing change, by selecting the Blue flag and alt-clicking somewhere else. However, it doesn’t copy the changed values contained therein.
(nb: this is across two separate Flows: I don’t know if that’s relevant.)
I guess it is relevant that you’re copying between different flows, for I have just copied several Note Spacing changes within the same flow and the values are preserved.
This is unfortunately to be expected for now. Because note spacing changes can be applied at the same rhythmic position in different frames showing the same music within the same flow, the way in which the data for the actual spacing values is stored inside them is very specific. I suspect that even copying and pasting them to a different frame chain but showing the same flow wouldn’t copy the values across, because the target won’t match (due to being in a different frame chain). I’m not sure what would be involved in trying to map these values, but I’m afraid it’s unlikely to be something we can devote time to in the immediate future.