Matrix has gotten much better about this. Most scenarios where device lists get out of date are caused by the device list cache for federared homeservers being

Matrix has gotten much better about this. Most scenarios where device lists get out of date are caused by the device list cache for federared homeservers being out of date. But with synapse 1.10 (released a few weeks ago), homeservers will refresh the device list much more aggressively in circumstances where it's clear that the cache is old (such as receiving a new message from a federated user on an unknown device).