Apparently, a fresh 32GB eMMC 5.x must be flashed via the procedure in Libre Computer AML-S905X-CC eMMC Flashing Steps from Linux. Using the procedure documented in this topic didn’t work (see my previous post).
I’m not sure if subsequent flashes (i.e. to change OSes) will work with this procedure now that one image has successfully been flashed. If someone knows the answer please let me know.