There is no dirty bit for modified properties in Unreal. The value is 'delta serialized', compared against the value in the archetype. If the value in the archetype is changed to a new value that is the same as some previously modified instances then those instances will save as 'no delta' / no change against their archetype.
Editor resets Per instance values if the parent was set to matching one
Result: The values are linked
Expected: Once modified, a variable is not linked to changes in default values
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-88414 in the post.