Why did my Rem appear in grey, and I had to click on it to be able to edit/expand it?

It’s not a big deal, and seems to be expected behaviour. However I’d like to understand why.

I was in a certain Rem, and noticed that some of the Rem within it were shown in grey (and became bold when I hovered on them). They weren’t editable (nor could be unfolded), until I clicked on them. After that, they behaved/looked as usual.

I think I’ve seen a similar question somewhere here or in Github, however couldn’t find it.

First time this happens to me…

For example, the Rem “Clearly Communicate” in the images below.
Before clicking:

After clicking: