After every server restart, all the custom head items that use the Texture tag and data lose this data. They still have other NBT tags like item name, but lose the texture. This affects all player head items stored in containers, inventories, or displayed on armor stands. The player heads placed as blocks are not affected, they keep the texture. This issue came up some time within the last 10 versions of Spigot, before that it hadn't been present.