‘Some’ of my highlights seem to have disappeared after leaving and returning to the app. I have noticed that if you click (hold) on one highlight that you have previously done, or just make a new one so you have something to select, it gives you forward and back arrows in a dialogue box which will allow you to sequentially step through your highlights. By doing this I have noticed that it will show a blue box around text that I have previously highlighted (as shown in the bottom of attached photo) but is not showing it highlighted in colour. It shows the highlight colour as opaque (indicated in the top right of the picture). You can then select this opaque colour option and choose any colour from a list and it will re-highlight it.

Super frustrating when you have highlighted hundreds of pages just so that you can quickly come back and read ‘highlighted’ key points only to find that they are not there and it becomes a laborious task to complete.