Lost Notes in pdf files in iBooks

Why do they disappear? I am reading a pdf book and I highlight words and sections and leave notes. Later, say the next day, I return to the book and the highlighting is there but the notes are gone. Curiously, if I change the color of the highlighting, the note reappears. This is really annoying. Why don't notes remain with the original color highlight? This happens on my iPad 4 and my iPhone 12. Also on my iPad Air 2