There was no warning when I first installed Signal that the usual phone backup mechanism (via iTunes, in my case) would not backup this data, or that mobile/desktop syncing problems might mean that hundreds of messages just don't get synced to my other devices, and that this is expected behavior.
So I'm angry, and I can't recommend Signal to others for this reason. And the devs just don't seem to care.
I appreciate that it's an open-source project, and the developers have no obligation to develop new features. But this isn't really a feature request; certain sharp edges in a product are actively destructive if you don't warn the user about them. It's like someone handing out free food which turns out to be poisonous, and then saying, "What are you all complaining about? The food is free!"
If any Signal developer sees this, I would personally be happy to have a discussion about compensating you for your time to fix this problem for the community. My email address is in my profile.