Missing glyphs in plots in plot pane (but not when saved via device)

I suddenly started having an issue where plots printed to the plots window show missing glyph symbols in the labels.
Plot in plots window:

Plot saved as PDF is normal

require(corrplot)
M<-cor(mtcars)
corrplot(M, method="circle")
pdf("test.pdf")
corrplot(M, method="circle")
dev.off()

I am using Rstudio 99.9.9 and R 3.6.2 on Clear Linux.

Update: It seems to happen with graphics devices other than PDF. The problem also occurs with png, jpeg, and svg devices. But pdf seems fine. It also happens when I run the code as an Rscript, so I don't think this is an Rstudio issue.