Okay, by digging around very unrelated websites, I discovered that by changing the xvfb-run line to
xvfb-run --server-args="-screen 0, 1280x1024x24" tiled --export-map $src/map.tmx map.lua
i was able to run this without segfaults. Don’t really know why this works but seems to be a color depth issue since the defaults are 1280x1024x8 and otherwise the same.