Probably also affects other backpack plugins, but my experience is specific to https://www.spigotmc.org/resources/shulker-backpacks-1-13-1-15.67466/ - Basically, opening a shulker pack directly from inventory and placing items inside results in item loss thanks to autosorting happening immediately on close, overwriting shulker contents. A small delay for pack to close first then sort could probably solve.