I've been receiving these warnings off-and-on for about a year. It's the strangest thing - they appear even after executing a command that doesn't even involve the offending variable, and persist across R sessions. The warnings appear randomly, so I haven't been able to reliably reproduce them.
The problem has reached a boil in the past two weeks because I'm automatically generating markdown reports, and many of them get spoiled by the output of these warnings into my chunk outputs.
The warning seems related to initializing a new column in a tibble. I have tried to inialize both with mutate() and with a direct assignment with $, but the warnings eventually start to appear either way.
My problem seems to be the same as the problem documented in this two threads.
Is anyone else plagued by these? Anything I can do to fix it? I am using RStudio Version 1.3.878. Many thanks!