Rmarkdown beamer slides does not show itemise bullets

I was trying to write create some beamer slides in Rmarkdown. Since the beamer slides would need to include some Chinese characters, I have to specify xelatex as the latex engine.

However, recently I found that the xelatex engine has a problem (the problem was not here a month ago): When using xelatex as the latex engine, the itemise bullets in beamer slides disappeared.

Does anyone know how to resolve this issue?

Below is a minimum working example:

---
title: trial doc
institute: |
  | Department trial
  | trial Education
  
Date: "17 October 2020"

output: 
  beamer_presentation:
    theme: "CambridgeUS"
    colortheme: "dolphin"
    slide_level: 3
    latex_engine: xelatex
    df_print: kable
classoption: "aspectratio=169"
fontsize: 10pt
mainfont: Times New Roman
    
   
header-includes:
  - \setbeamercolor{frametitle}{bg=white}
  - \usepackage{ctex}
  - \AtBeginSubsection{}
  - \AtBeginSection{}
  - \setbeamerfont{frametitle}{series=\bfseries}
  - \AtBeginDocument{\title[xxx]{xxxxx}}
  - \AtBeginDocument{\institute[xxxxx]{\\Department of xxx\\xxx}}
  - \AtBeginDocument{\author[xxxxxx]{XXX}}
  

  
---

\frametitle{Outline}

\tableofcontents

# 欢迎

\frametitle{\textbf{Welcome}}

some text

some Chinese text 你好

trying to itemize:

- Hello
  - subitem

trying some numbered list:

1. hello

2. hello

And the output I get is as below:

Output from the code above

When I try using pdflatex as the latex engine (in which case I cannot use the ctex package and cannot include Chinese characters in my beamer slides), the itemize and numbered list works perfectly.

Really appreciate if someone can help me with this!