One of scenarios: when you bring head of zombie/creeper/skeleton/ender-dragon/human and put it on anvil together with any enchanted book, glowing effect doesn't show up in inventory and also when you wear it.
In conclusion: skulls(heads) don't have "glowing effect" when enchanted.
This bug is very old, i just marked the latest Minecraft version.
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kumasasa
@unknown: Watch your language.