How is the rendering backend selected for ggplots?

I am using a shiny application with a ggplot which contains a unicode alpha character (\u03b1), something like

gg <- plot_data %>%
ggplot2::ggplot(ggplot2::aes(x = x)) +
ggplot2::annotate(
"text",
x = 5,
y = 100,
color = "blue",
size = 5,
label = "alpha \u03b1"
)

The alpha is correctly rendered on my mac, but it's not rendered on the deployment platform (linux centos). Similarly, fill colors are present on my mac, and absent on linux. Everything else looks fine.
I am running with the exact same version of shiny (1.4.0) as well as all other dependencies, so I suspect that the difference emerges from the installed system libraries (e.g. cairo) and how they are chosen to perform the actual rendering.

How is the rendering backend selected in shiny, and what am I missing on the linux machine so that it renders properly?

I tested on my windows, and on my Linux machine, and saw the plot with no problem...

but I did find this blog, which indicates it might relate to available fonts
https://michaelbach.de/2020/03/22/ggplotAndUnicode.html

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