In general we try not to delete data that you might still find useful. The casting off could change in a transitory fashion because of an edit elsewhere on the system. If you then made a further edit such that the casting off goes back to how it was before, wouldn’t you then be equally perturbed to find that the edit had been removed, even though now the casting off has gone back to how it was before?
The simple fact is this: the property is stored relative to the segment of the hairpin, even if there is only one segment. If another segment comes into being, Dorico does not remove properties from the first segment (this would be deleting data that can still apply, e.g. would you want it to remove the vertical offset in such a circumstance? of course you wouldn’t!).
I don’t think this is unreasonable behaviour, nor do I think it is contrary to the general philsophy of how the program operates, which is that wherever possible we will retain edits you have made in Engrave mode unless you explicitly remove them.