In latex output, pandoc does not put a comment character after \CSLLeftMargin{ ... }, which results in \CSLLeftMargin{ ... } and \CSLRightInline{ ... } effectively being separated by a space char.

Since the widths of \CSLLeftMargin and \CSLRightInline are defined as \csllabelwidth and \linewidth - \csllabelwidth, their combined width including the separating space amounts to \linewidth plus the width of a space char, resulting in a line wider than \linewidth.

Running pandoc -s -C test.md -o test.pdf does place the \CSLLeftMargin and \CSLRightInline boxes on the same horizontal position in the pdf, but the \CSLRightInline box extends into the right margin (and pandoc -s -C test.md -o intermediate_tex_file.tex followed by pdflatex intermediate_tex_file.tex shows that pdflatex complains about Overfull \hbox (2.22221pt too wide) in paragraph at lines 85--91).

In beamer output (pandoc -s -C test.md -t beamer -o test.pdf), the effect is more drastic: Here, the \CSLLeftMargin and \CSLRightInline boxes appear on different lines, as shown below:

Expected layout:

Suggested solution: Either add a comment character after \CSLLeftMargin{ ... } to prevent a space from appearing between the \CSLLeftMargin and \CSLRightInline boxes, or reduce the \CSLRightInline width by modifying the definition to something like

\newcommand{\CSLRightInline}[1]{\parbox[t]{\linewidth - \csllabelwidth - \fontdimen2\font}{#1}\break}

(The simplest method for specifying the width of the space char of the active font seems to be to use \fontdimen2\font; see https://tex.stackexchange.com/questions/30746/how-does-tex-know-how-to-space-different-typefaces and https://tex.stackexchange.com/questions/390651/how-to-calculate-the-width-of-a-space.)

Or use explicit widths, defining

\newcommand{\CSLRightInline}[1]{\parbox[t]{\linewidth - \csllabelwidth - 1ex}{#1}\break}

and outputting

\CSLLeftMargin{ ... }\hspace{1ex}%
\CSLRightInline{ ... }

test.md (with examples for two LeftMargin/RightInline CSL styles, and not using section headers to keep all text on one beamer slide):

---
csl: nature.csl
# csl: vancouver.csl
references:
- id: olson:2006codes
  author:
    - family: Olson
      given: Hope A.
  container-title: Library Quarterly
  DOI: 10.1086/504343
  issue: '1'
  issued: 2006
  language: en-US
  page: 19-35
  title: 'Codes, costs, and critiques: The organization of information in *Library
    Quarterly*, 1931–2004'
  type: article-journal
  volume: '76'
...

Blah [@olson:2006codes]. Blah blah. And some more text just to show the width of a normal paragraph.


References

pandoc 2.17.0.1 / macOS 11.6.2