I have now seen this a few times. I’m editing something, like a forum post, and I have entered the advanced editor so I can adjust some links in the post. If a link exists, sometimes, if I try to highlight (or even just click on) the linked text, Moodle immediately kicks me out of the Advanced Editor (meaning that I can no longer edit the link). The workaround that I found was to highlight more of the text than just the link and then click the button for Remove Link. Then, I can highlight just the text I want and put a new link back in. But this is very frustrating when I am quickly editing something and keep getting bumped out of the Advanced Editor.

The video below demonstrates this behavior. 

Moodle Advanced Editor Kicks Me Out