we found that the latest versions of iBooks (for both iOS and macOS) changed the behaviour of the read aloud playback in a way that may skip the highlighting of the first block if this is marked to be highlighted as soon as the audio starts (0.00 secs). This may happen if you pressed the space bar - in the read aloud wizard - before the audio actually started playing.
To fix this, please wait to actually hear the voice before pressing the space bar for the first time in the read aloud wizard. This will mark the first block to be highlighted at a time > 0.00 secs (even 0.001 secs would be ok!) and everything should work.
Please let me know if you have any further questions.