I just updated R to version 3.5, then I don't really know what I did that the outputs obtained after I execute a chunk changed. So now the outputs look like the ones in the console, see the before and after examples for the following code.
I suggested a possibly related topic over in that thread, but I don't know for sure if the issues are the same.
Anyway, as far as what to google, I believe the problem is that the auto-rendering RStudio IDE usually does for some types of R Notebook chunk output isn't happening, so you're getting the plain old console output instead. (And note that the IDE now treats all R Markdown documents as Notebooks unless you tell it not to). You might consider moving this post to the RStudio IDE category, to see if it gets more attention there.