This has occurred on MacBook Safari + Chrome, iOS and iPhone and iPad app.
When playing audio (through generate audio) from the beginning and paging through, the underline moves to each sentence with the audio. Good.
However, the moment you start skipping, the further away you skip, the more delay there is between the beginning of the sentence and when it underlines it.
This should be easily replicable by simply skipping through after generating audio in a longer lesson. Another way to do it would be good into the sentence by sentence mode (expanding the generate audio) and clicking on a sentence far down and listen for where it starts in the sentence and when it highlights the next one.
This bug is heavily impacting what I would argue is a strong feature of the software and I urge you to attempt fixing as soon as possible.