Whenever I get a custom Spawner VIA commands, it spawns the mob once, then reverts to a pig spawner.
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/give @p minecraft:spawner{BlockEntityTag:{SpawnCount:10,SpawnRange:5,Delay:0,MinSpawnDelay:0,MaxSpawnDelay:0,MaxNearbyEntities:10,RequiredPlayerRange:20,SpawnData:{id:"minecraft:zombie",CustomNameVisible:1b,CustomName:"{\"text\":\"Zombie Guard\",\"color\":\"dark_red\",\"bold\":\"true\"}",HandItems:[{id:"minecraft:iron_sword",Count:1b,tag:{display:{Name:"{\"text\":\"Heavy Blade\",\"color\":\"yellow\"}",Lore:["A massive blade carried by","the guards of Higum's prison.","You feel slower holding it."]},HideFlags:2,Enchantments:[{id:"minecraft:sharpness",lvl:2}],AttributeModifiers:[{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:-0.3,Operation:1,UUIDLeast:888993,UUIDMost:622703,Slot:"mainhand"},{AttributeName:"generic.attackDamage",Name:"generic.attackDamage",Amount:2,Operation:2,UUIDLeast:382197,UUIDMost:873479,Slot:"mainhand"}]}},{}],HandDropChances:[2.0F,0.85F]}}} 1
Most of this is flavor Text
What was the command you used?