To reproduce:

1. Starting with Minecraft 1.16.5, install Fabric loader and Fabric API.
2. Install mobdisguises-1.0.0-fabric and disguiselib-1.0.2-fabric.
3. Create a new world with cheats enabled.
4. /summon minecraft:iron_golem ~ ~ ~ {CustomName: '""', DisguiseLib: {DisguiseType: "minecraft:player", DisguiseAlive: 1b}, UUID: [I;-873554585,-1697035970,-1367762496,1015370517]} (summons an Iron Golem disguised as a player)
5. /summon minecraft:iron_golem (summons an undisguised Iron Golem)
6. Any of the following:
   /give @p flint_and_steel
/give @p bow then hold the bow and /enchant @p flame
/give @p wooden_sword then hold the sword and /enchant @p fire_aspect 2
7. Light both Iron Golems on fire, then observe the resulting behavior:

• The undisguised Iron Golem is properly set on fire.
• The disguised Iron Golem is set on fire server side, but this isn't shown to the client.

Should be fixed in 1.0.4