The problem of your setup is that the RT1<->RT2 switching inside the encoder is not synchronized with the Magic RDS External text sources. The External text sources tool does not control Radiotext 2. The Radiotext 2 is deprecated feature and should be entirely disabled if the encoder is permanently online.
Use the setup proposed here - it is simple to realize and the result is perfect:
https://pira.cz/forum/index.php?topic=990.0If you add the Text Campaigns and inline RT+ for fixed text messages, you'll bring your RDS to another level.