Bug description
I've added mobs by tags into the mod's flying.json file, yet I'm still unable to fly with them. Whilst on survival. Both "enableFlight" is set to true and no achievements are meant to be needed to be able to fly (I'm able to fly with vanilla mobs, however modded, after added to the mod's .json won't fly).

I'm dropping my flying.json and my identity.json.

identity.json
flying.json

Don't do that. You need to create a datapack which overrides the data of identity.
I think there are plenty tutorials on YouTube, but here's an official guide: https://minecraft.fandom.com/wiki/Data_pack

You forgot to add a , after each line. (Except the last one)

Here's a fixed version of your flying.json:

{
  "replace": true,
  "values": [
    "minecraft:ghast",
    "minecraft:bat",
    "minecraft:blaze",
    "minecraft:parrot",
    "minecraft:phantom",
    "minecraft:bee",
    "minecraft:ender_dragon",
    "minecraft:wither",
    "minecraft:vex",
    "minecraft:allay",
    "animalistic_a:murcielago",
    "alexmobs:crow"
  ]
}

You forgot to add a , after each line. (Except the last one)

Here's a fixed version of your flying.json:

{
  "replace": true,
  "values": [
    "minecraft:ghast",
    "minecraft:bat",
    "minecraft:blaze",
    "minecraft:parrot",
    "minecraft:phantom",
    "minecraft:bee",
    "minecraft:ender_dragon",
    "minecraft:wither",
    "minecraft:vex",
    "minecraft:allay",
    "animalistic_a:murcielago",
    "alexmobs:crow"
  ]
}

Hi! Thx for ur help. I've tried this and I'm still not able to fly with neither of these mobs. It's important I mention I'm pulling the .jsons out of the java file with WinRAR and putting it back after edited. Still, I cannot fly with these mobs.

I have noticed I forgot an "s" after "alex". However, Animalistic's Alpha Bat is meant to be able to fly, but isn't after added like this.

Don't do that. You need to create a datapack which overrides the data of identity. I think there are plenty tutorials on YouTube, but here's an official guide: https://minecraft.fandom.com/wiki/Data_pack

Oh God. That will take a bit. Thank you very much! I'll try that see how it goes