The column label does have the gray lines that the text should appear between.
When I uncheck "Formatting in comments" the comment text appears between the lines.
When I go back into settings and check "Formatting in comments" the comment text disappears but the lines are still there.
I tried looking after opening and using Chrome for a second as you suggest but it didn't change anything.
I also tried checking and unchecking "formatting in comments" and the "comments use full width" boxes but that also didn't change anything.
Here's something else too: I also run CC on my Kindle Fire tablet and the comments are showing up there with the formatting in comments box checked.
I have the whole library on it just like I do on my phone and it's updated with all books as of today, same as the library on my phone. It uses the same CC app because I use amazon app store to put it on all my android devices. Version 5.4.3.3 (391)
The thing about the Android WebView and this issue that doesn't make sense to me is that my nook has been in airplane mode for years because I do all my reading on it from sideloaded books. There's no android updates happening there. The missing comments just started on the nook, too.
I definitely am thinking something in Calibre did it. I just can't figure out if I did something that triggered a glitch that won't reverse itself or if it's something else. And, honestly, I guess there is a chance that it's different issues that just happened to occur around the same time. Seems remote, but weirder things have happened to me.