Giants equipped with a skull, head, or block (Eg. Player Heads, Skeleton Skulls, Creeper Heads, Carved Pumpkins, etc.) as a helmet are unable to render them when equipped. However, they are able to render helmets.

Replication:
1. Summon a Giant with appropriately attached NBT data, so that it is equipped with non-rendering headgear. Some examples:

Example 1:

/summon minecraft:giant ~ ~ ~ {ArmorItems:[{},{},{},\{id:"minecraft:carved_pumpkin",Count:1b}]}

Example 2:

/summon minecraft:giant ~ ~ ~ {ArmorItems:[{},{},{},\{id:"minecraft:skeleton_skull",Count:1b}]}

Example 3:

/summon minecraft:giant ~ ~ ~ {ArmorItems:[{},{},{},\{id:"minecraft:creeper_head",Count:1b}]}

Additional Information/Verification:
The items are successfully equipped, as setting the drop rate for the respective items to 100% will result in a successful drop upon the Giant's death.

/summon minecraft:giant ~ ~ ~ {ArmorItems:[{},{},{},\{id:"minecraft:skeleton_skull",Count:1b}],ArmorDropChances:[0.00F,0.00F,0.00F,1.000F]}

Tested on Minecraft: Java Edition on versions 1.15.2 and 1.16-snapshot-20w13b