Christophe,
You were right. Your suggestion fixed the problem.
I didn't change any R packages or hugo, but I did upgrade macOS. Then, even the latest R packages and hugo had the same problem. Thanks for providing the link to the StackOverflow post. Since I'm not calling hugo directly, but rather through blogdown::build_site(), the actual fix I used was
options(blogdown.hugo.args = "--noTimes")
I really appreciate you help in diagnosing the problem and finding a fix.