I think they're probably doing what they did for the Clones, have a set actor for live action and TV. Casting an additional person to play the young Rosario Dawson version prevents that from getting muddied.

Sad that we didn’t get Ashley dubbed as young Ahsoka

To be fair, your voice sounds different to you than when other people hear it, because of our sinuses, etc. I imagine it's worse for Ahsoka with those montrals and lekku covering up her ears.