bookdown: caption style is not rendered

I've tried to customize captions using expressions as described in this post.

While the post stated that Markdown syntax is supported, I could not make a strong type. Instead of strong type, in the book, I got caption enclosed with asterisks, like **Table 1.1.**. I reproduced this behavior by copying Yihui's minimal bookdown and adding _bookdown.yml to it.

index.Rmd

---
title: "reprex"
site: bookdown::bookdown_site
documentclass: book
output:
  bookdown::gitbook: default
---

# Hello World

```{r}
knitr::kable(
  head(iris),
  caption = 'A table generated by the longtable package.'
)
```

_bookdown.yml

language:
  label:
    tab: !expr function(x) sprintf("**TABLE %s.** ", x)

output:

Any suggestions why it does not work?