Thanks jcblum for your quick response. It is possible my problem is the same issue.
I tried to post a .nb.html file with the problem (and planned to post the corresponding .html), but restrictions again: "Sorry, the file you are trying to upload is not authorized (authorized extensions: jpg, jpeg, png, gif, pdf)."
I was able to create a simple example that shows the same problem. (I used dput on the original object that was not displaying correctly).
y <- structure(list(n = 768L), row.names = c(NA, -1L), class = c("tbl_df",
y does not display correctly in a notebook, but looks fine in a document.
It's possible this problem is related to the new R 3.5.0.
I'm not seeing the same problem with R 3.4.4 on a different machine. (When I tried to reconfigure RStudio to use R 3.4.4, which is still present, I get an error that appears to be connected to the Rcpp.dll.)