Strange behaviour of do.call()

The knitr::kable() function does some internal setup, including 
determining the target format, and then calls an internal function using

? do.call(paste("kable", format, sep = "_"), list(x = x,
??????? caption = caption, escape = escape, ...))

I was interested in setting the `vlign` argument to 
knitr:::kable_latex, using this code:

? knitr::kable(head(mtcars), format="latex", align = "c", vlign="")

If I debug knitr::kable, I can see that `vlign = ""` is part of 
list(...).? However, if I debug knitr:::kable_latex, I get weird results:

? > debug(knitr:::kable_latex)
? > knitr::kable(head(mtcars), format="latex", align = "c", vlign="")

debugging in: kable_latex(x = c("Mazda RX4", "Mazda RX4 Wag", "Datsun 
710",
? "Hornet 4 Drive", "Hornet Sportabout", "Valiant", "21.0", "21.0",
? "22.8", "21.4", "18.7", "18.1", "6", "6", "4", "6", "8", "6",
? "160", "160", "108", "258", "360", "225", "110", "110", "93",
? "110", "175", "105", "3.90", "3.90", "3.85", "3.08", "3.15",
? "2.76", "2.620", "2.875", "2.320", "3.215", "3.440", "3.460",
? "16.46", "17.02", "18.61", "19.44", "17.02", "20.22", "0", "0",
? "1", "1", "0", "1", "1", "1", "1", "0", "0", "0", "4", "4", "4",
? "3", "3", "3", "4", "4", "1", "1", "2", "1"), caption = NULL,
????? escape = TRUE, vlign = "")
debug: {

? [rest of function display omitted]

I see here that vlign = "" is being shown as an argument. However, 
when I print vlign, sometimes I get "object not found", and somethings 
I get

? Browse[2]> vline
? debug: [1] "|"

(which is what the default value would be).? In the latter case, I 
also see

? Browse[2]> list(...)
? $vlign
? [1] ""

i.e. vlign remains part of the ... list, it wasn't bound to the 
argument named vlign.

I can't spot anything particularly strange in the way knitr is 
handling this; can anyone else?? My sessionInfo() is below.

Duncan Murdoch

sessionInfo()

R version 4.3.1 (2023-06-16)
Platform: x86_64-apple-darwin20 (64-bit)
Running under: macOS Monterey 12.6.9

Matrix products: default
BLAS: 
/Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/lib/libRblas.0.dylib 

LAPACK: 
/Library/Frameworks/R.framework/Versions/4.3-x86_64/Resources/lib/libRlapack.dylib; 
?LAPACK version 3.11.0

locale:
[1] en_CA.UTF-8/en_CA.UTF-8/en_CA.UTF-8/C/en_CA.UTF-8/en_CA.UTF-8

time zone: America/Toronto
tzcode source: internal

attached base packages:
[1] stats???? graphics? grDevices utils???? datasets? methods base

loaded via a namespace (and not attached):
[1] compiler_4.3.1 tools_4.3.1??? knitr_1.44???? xfun_0.40