Sorry to hijack your thread, but this is somewhat related.
sourceRectAtTime works as expected when typing up the text and also if I vary the length of each line on a multi-line text layer, but if I animate the writing of the text using a text animator (typewriter, for example) it only shows the end size, the affected object does not vary its size as the text layer is growing in size.
Your help is greatly (and urgently) appreciated.