Unable to compile Rmd file with external tikz file - r

Question

I'm trying to compile a pdf file from an Rmd document with some tikz graphics in it.
Example Rmd:
---
output:
pdf_document:
latex_engine: lualatex
header-includes:
- \usepackage{tikz}
---
# Introduction
Hello.
```{tikz img-plot, echo = FALSE, engine = "tikz", fig.ext = "pdf", fig.cap = "This is my image", code = readLines("testimg.tex")}
```
Figure \#ref(fig:img-plot) is not working.
referenced tikz file testimg.tex:
\begin{tikzpicture}
\node at (0.5, 0.5) {(a)};
\end{tikzpicture}
Compiling the Rmd file in Rusing
$ echo "rmarkdown::render('test.Rmd', output_file='test.pdf')" | R --no-save -q
gives me the following log:
rmarkdown::render('test.Rmd', output_file='test.pdf')
processing file: test.Rmd
|
|
| 0% |
|.......................
| 33% ordinary text without R code
|
|...............................................
| 67% label: img-plot (with options) List of 5 $ echo : logi
FALSE $ engine : chr "tikz" $ fig.ext: chr "pdf" $ fig.cap: chr
"This is my image" $ code : language readLines("testimg.tex")
|
|......................................................................|
100% ordinary text without R code
output file: test.knit.md
/usr/bin/pandoc +RTS -K512m -RTS test.utf8.md --to latex --from
markdown+autolink_bare_uris+tex_math_single_backslash --output
test.tex --template
/usr/lib/R/library/rmarkdown/rmd/latex/default-1.17.0.2.tex
--highlight-style tango --pdf-engine lualatex --variable graphics=yes --lua-filter /usr/lib/R/library/rmarkdown/rmd/lua/pagebreak.lua --lua-filter /usr/lib/R/library/rmarkdown/rmd/lua/latex-div.lua --variable 'geometry:margin=1in' --variable 'compact-title:yes' ! error: (file "test_files/figure-latex/""img-plot-1".pdf) (pdf
backend): canno
Error: Failed to compile test.tex. See
https://yihui.name/tinytex/r/#debugging for debugging tips. See
test.log for more info. Execution halted
The error log shows a double quote in ""img-plot-1".pdf. Could this have something to do with it?
I used to be able to compile a similar file a few weeks ago, which is why my guess is that some update in an R package, pandoc or somewhere else on my system broke it. Any ideas what the root of this might be?
System info:
OS: Linux Kernel 4.19.85-1-MANJARO
R 3.6.1
rmarkdown package: 1.17
pandoc 2.7.3