It may happen in some other scenarios, too, I'm not sure yet.
If it is intended, is there any way around it? That's not the kind of file where you'd necessarily want this to happen. Sure, you want the changes saved, but if there are pre-existing things in the file that are unrelated to what's being saved (which is the case), shouldn't they be retained?
Something like this, for example, will go:
Thankspush "dhcp-option DNS 192.168.1.100"