Thanks for sharing all this. After a quick step by step debugging, I found that the issue was not related to R Markdown but with an entry in your bib file. The only reference causing issue is @lg which I think as an incorrect syntax because using 2 like that : {{ }} in the title field
@article{lg,
author = {Leenders, C. and G. Geerdink},
journal = {Tijdschrift voor Lerarenopleiders},
number = {3},
pages = {55--66},
title = {{Mannelijke pabostudenten kiezen niet voor kleuters. Weloverwogen of op basis van misconcepties? }},
volume = {34},
year = {2013}
}
If you remove the duplicated { and }, I believe it will work as expected.
It could be an issue with Pandoc new citeproc that does not throw an error and hand. Because with old Pandoc 2.7.3 it works ok, but with new version shipped in RStudio, it does not. You can as in Pandoc discussion (https://groups.google.com/g/pandoc-discuss) using a minimal example without rmarkdown.
For example in test.md
---
title: "Studievoortgang aan de pabo"
bibliography: references.bib
csl: https://www.zotero.org/styles/apa-6th-edition
---
studies van Gerda Geerdink waarvan een meer recente @lg
with the faulty bib file and using this commandline
pandoc -t html -f markdown --citeproc test.md
Hope it helps