At https://github.com/sandflow/imscJS/blob/master/src/main/js/html.js#L1433-L1458, tts:direction is used to determine the mapping of tts:textAlign and not context.ipd. TTML2 says:

The tts:textAlign attribute is used to specify a style property that defines how inline areas are aligned within a containing block area in the inline progression direction.

Is the current code correct?