I upgraded Rstudio to 1.2 in Sep 2019. I have noticed that some of the dataframes loses the blue dropdown button which showes the column (equivalent of "str" command) when I use some of the dplyr functions.
I have asked this question on stackoverflow as well but have not had an answer yet. its annoying more than anything else.
It was fixed in a preview version of RStudio 1.2 so I'd recommend updating to the latest build (1.2.5019) and check. If that doesn't fix it, then you might want to raise a bug report with RStudio.