Is this a bug? expression(mu) appears as a proportional-to symbol instead of as mu in Rmarkdown

Question

This is my minimal code example that knits on my computer (Windows 10, R 4.3.1, latest version of knitr and tinytex) but shows a proportional symbol where the mu should be.

---
title: "temp"
output: pdf_document
date: "2024-02-26"
---


## Including Plots

You can also embed plots, for example:

```{r}
plot(1:2,1:2)
mtext(expression(mu), at=1.5, line=-2)
```

And this is a screenshot of the output:

Is this a bug in Rmarkdown? If not, what can I do to fix it? Thanks.