Thanks to @yuk's comments, I was able to narrow the problem down to the textshaping package (a dependent of ragg, which is a dependent of officer, which is a dependent of flextable). Upon a closer reading of this thread (How to unload a package without restarting R), I realized that unloadNamespace wasn't doing everything I needed. Instead, plugging in pkgload::unload("textshaping") before the plot functions solved the issue. Including library(textshaping) after I've rendered the charts puts it back in place just fine for when I need it later.