Certain types of content (lists, headings, titles, indented quotes) generally cannot be highlighted - I recommend you try a TG and a BD entry to see - what you experience with one (of each, as the TG could behave differently from the BD), you will likely experience with all.
This is a known issue with the content (not the app), and some of it has been updated over the course of years to allow highlighting on that sort of thing (e.g. in new manuals), but there's a ton of content, so it will take time. Since they know about the issue and how to "fix" it, I recommend praying for blessings on those who are involved in this work, to aid them in their progress.
In the meantime, I put my highlights / notes as close to the text in question as possible, and include in my note a reference back to what I wanted to highlight (and if it's short, I might copy the text and paste it into my note).
FWIW,
Liz