When the metadata is updated, the IDs can change. The metadata is only published on change (with retain=true). That means, when you subscribe to it and receive an update, you need to update your internal name to ID map.
There is a field called “mdHashVer” which is currently only implemented by the PROFINET IO Connector. It should be also implemented by other connectors in future versions. The "mdHashVer" field of the "dpValue" payload must match the "hashVersion" number in the metadata definition.
This is a synchronized way to know exactly which metadata version was used for which data message because the update of the metadata and data of MQTT topics is asynchronously and the notification order between Metadata message and Value message might be reversed (see also the sequence diagram below).