Dehydrated devices should never generate to-device messages, so incoming to-device messages from a dehydrated device indicate a faulty implementation. We have decided to drop such messages, per [the MSC](https://github.com/uhoreg/matrix-doc/blob/6be907845c1dae03410a4c17f08d601e4dab5c56/proposals/3814-dehydrated-devices-with-ssss.md?plain=1#L53-L55)
