It definitely appears on reddit when you click on the comment thread, which will force you to download it separately. It was also on sites other than reddit, where there are no comments or a transition to another page.

Select a piece of text in the article (let's call it #1), everything is ok, it's highlighted in color, and it's in the notes menu. Select the text #2, and it is marked with a color, but it is not in the menu. In addition, the #1 text highlighting disappears, even though it is in the menu. After restarting the app, the text #1 is both highlighted and in the menu, but the text #2 is not selected and is not in the menu.

And this is happening again. If you manage to select text #2 after rebooting, selecting text #3 will cause the same bug with respect to text #2 as highlighting #2 caused #1 in relation to text: #2 will lose color, but will remain in the menu, text #3 get color but will not appear in the menu. After reloading, text #2 is fine, text #3 is not selected and is not marked.

Something similar was/is in the firefox extension on PC. It's the same there, but just reload the page.

For the problem with the extension, the fix worked:

I tested it on two smartphones. There was also a bug before the last update.