`xelatex` rendering issue on shinyapps.io

Hello everyone. Even though, according to the latest posts, this issue (#205) seems to be solved on shinyapps.io, from my shiny app (deployed there) I am still not able to produce a PDF from an .Rmd file embed in the application if the version of the rmarkdown package is higher than 1.15 and the latex engine is xelatex.

When I have this version installed, everything works fine, even though I get the following warning when the application starts:

This version of bslib is designed to work with rmarkdown version 2.7 or higher

If, however, I try to install the latest version of rmarkdown my PDF report is not produced and I get the following LaTeX error:

! LaTeX Error: Missing \begin{document}.
Warning: Error in : LaTeX failed to compile /var/folders/f9/8lfgdj3d5s1dbzsbtscxg9w80000gp/T//RtmpWVpFWz/file10d19ca31140.tex. See https://yihui.org/tinytex/r/#debugging for debugging tips. See file10d19ca31140.log for more info. [No stack trace available]

Any idea on how to solve this? Thank you for your help!

Originally posted by @nicocriscuolo in https://github.com/yihui/tinytex/issues/205#issuecomment-864039152

Reproduce the PDF generation from an Rmd app on shinyapps.io with rmarkdown above 1.15 and the xelatex engine. Inspect the generated .tex file and its LaTeX log, using the reported “Missing \begin{document}” failure and issue #205 as context; done means the PDF renders successfully.