Minecraft Version 1.18.1
Origin Version 1.3.11
Example Conditioned Attribute Modifier:

    "type": "origins:conditioned_attribute",
    "modifier": {
      "name": "Speed reduction",
      "attribute": "minecraft:generic.movement_speed",
      "value": 1,
      "operation": "addition"
    },
    "condition": {
      "type": "origins:biome",
          "condition": {
            "type": "origins:category",
            "category": "desert"
         }
    }

The modifier stacks every 20 ticks rather than just apply once

changed it to "multiply_base" and the issue went away, I will use that for now, just strange that "addition" is stacking for me. (No mods other than JEI added)

I can't actually replicate it. As per usual, /reload breaks power, in this case the prior modifier stays active, and a new one is applied, but I can't replicate it during actual gameplay.

I need a WTF label gimme a sec.

hmm... ya I have not used the /reload command. I will continue testing