Special character does not show up in bibliography

Question

I am writing my dissertation with bookdown (https://bookdown.org/yihui/bookdown/) and the memoir class in R. I need to include special characters such as "\oe". This character appears correctly in the body of the document (therefore it's NOT a font problem).

However, the same character does not appear in the bibliography section. More precisely, nothing appears instead of the special character. The .bib file is a .tex generated by Zotero where the "\oe" symbol (for instance) is represented as "Å".

I have disabled citation packages in order to format the bibliography section following a specific citation style (i.e., I am using the apa6.csl).

It is difficult to provide a reproducible example though, as I am working from my messy dissertation repository. But here is (a part of) my _output.yml file:

bookdown::pdf_book:
  includes:
    in_header: latex/preamble.tex # defines style and latex options
    before_body: latex/before_body.tex # defines cover page
  latex_engine: xelatex # lualatex or xelatex
  citation_package: none # needs to be "none" in order to use the csl file
  keep_tex: true # keeps the .tex file
  dev: "cairo_pdf"
  toc: false # deactivates default table of contents
  highlight: pygments # code highlighting
  pandoc_args: [ "--csl", "./csl/apa6.csl" ] # specifies the csl file to be used

Any idea ?