I have been testing this. 
The images I grabbed from the example topic in the OP work fine on my test site, but comparing the code in the browser inspector, mine get a lightbox-wrapper and @3DPB’s do not. I’ve tried tweaking a few settings, but no effect so far.
You could check your create thumbnails and composer media optimization image enabled admin settings, just to be sure (and confirm that we have a similar set up for any tests )
The only other topic I could find with a similar issue is this one, but the solution was to dial up a new instance which magically fixed it, so the same solution that’s been offered already.
So far, it’s quite a frustrating problem to replicate/solve.