It just doesn't set the value even if the command doesn't fail.

To reproduce:

1. Summon a creeper near you /summon creeper ~ ~ ~ {NoAI:1}
2. Add the modifier to the creeper: /data modify entity @e[type=creeper,limit=1,sort=nearest] Attributes[ {Name:"minecraft:generic.max_health"}].Modifiers append value
   {Name:"test.max\_health",Amount:0.0,Operation:1,UUID:[I;-633757882,1012614665,-1758719176,-1591490096]}
3. Check that the modifier is there: /data get entity @e[type=creeper,limit=1,sort=nearest] Attributes[ {Name:"minecraft:generic.max_health"}].Modifiers ... it's there
4. Add a dummy scoreboard objective /scoreboard objectives add test dummy
5. Set a score value for the creeper /scoreboard players set @e[type=creeper,limit=1,sort=nearest] test 16
6. And now the offender: /execute as @e[type=creeper,limit=1,sort=nearest] store result entity @s Attributes[ {Name:"minecraft:generic.max_health"}].Modifiers[
   {Name:"hg.max_health"}].Amount double 1 run scoreboard players get @s test ... The command outputs entity_uuid has 16 [test] but the Amount is not set:
7. /data get entity @e[type=creeper,limit=1,sort=nearest] Attributes[ {Name:"minecraft:generic.max_health"}].Modifiers