Loot Table issue with leaves

If you have the registry name correct, it's possible the loot table is in the wrong folder for some reason. I had a similar issue with certain recipe overrides being put in the mod data file instead of the minecraft data file. (And therefore not working.) You could check using the built in file explorer in the left tab of the workspace, or by manually looking in the mod's src folder. The data folder should contain a separate file for minecraft and your mod's data, and you need your recipe to be in the minecraft folder for the override to work- MCreator normally does this automatically, but it's for some reason inconsistent at times.

...And of course if that fails, you could also just forget it and make a procedure that has a chance of dropping the item whenever the block is broken. It'll amount to the same thing, even if it's not the most elegant solution...

I think leaves may have special loot tables for this very reason- they need to have different pools depending on whether the block is mined with shears, whether it decays, etc; not necessarily just silk touch. Here's the vanilla azalea-leaves loot table for reference, not sure what to make of it:

{
  "type": "minecraft:block",
  "pools": [
    {
      "bonus_rolls": 0,
      "entries": [
        {
          "type": "minecraft:alternatives",
          "children": [
            {
              "type": "minecraft:item",
              "conditions": [
                {
                  "condition": "minecraft:any_of",
                  "terms": [
                    {
                      "condition": "minecraft:match_tool",
                      "predicate": {
                        "items": [
                          "minecraft:shears"
                        ]
                      }
                    },
                    {
                      "condition": "minecraft:match_tool",
                      "predicate": {
                        "enchantments": [
                          {
                            "enchantment": "minecraft:silk_touch",
                            "levels": {
                              "min": 1
                            }
                          }
                        ]
                      }
                    }
                  ]
                }
              ],
              "name": "minecraft:azalea_leaves"
            },
            {
              "type": "minecraft:item",
              "conditions": [
                {
                  "condition": "minecraft:survives_explosion"
                },
                {
                  "chances": [
                    0.05,
                    0.0625,
                    0.083333336,
                    0.1
                  ],
                  "condition": "minecraft:table_bonus",
                  "enchantment": "minecraft:fortune"
                }
              ],
              "name": "minecraft:azalea"
            }
          ]
        }
      ],
      "rolls": 1
    },
    {
      "bonus_rolls": 0,
      "conditions": [
        {
          "condition": "minecraft:inverted",
          "term": {
            "condition": "minecraft:any_of",
            "terms": [
              {
                "condition": "minecraft:match_tool",
                "predicate": {
                  "items": [
                    "minecraft:shears"
                  ]
                }
              },
              {
                "condition": "minecraft:match_tool",
                "predicate": {
                  "enchantments": [
                    {
                      "enchantment": "minecraft:silk_touch",
                      "levels": {
                        "min": 1
                      }
                    }
                  ]
                }
              }
            ]
          }
        }
      ],
      "entries": [
        {
          "type": "minecraft:item",
          "conditions": [
            {
              "chances": [
                0.02,
                0.022222223,
                0.025,
                0.033333335,
                0.1
              ],
              "condition": "minecraft:table_bonus",
              "enchantment": "minecraft:fortune"
            }
          ],
          "functions": [
            {
              "add": false,
              "count": {
                "type": "minecraft:uniform",
                "max": 2,
                "min": 1
              },
              "function": "minecraft:set_count"
            },
            {
              "function": "minecraft:explosion_decay"
            }
          ],
          "name": "minecraft:stick"
        }
      ],
      "rolls": 1
    }
  ],
  "random_sequence": "minecraft:blocks/azalea_leaves"
}

That's the trouble- I don't know if there is a way to detect when leaf blocks decay, unless you've custom coded the leaf behavior yourself for a modded block. Microsoft has some basic documentation on loot tables, maybe that'll help?

It is for bedrock, but for some reason most of the syntax is the same. You could also try finding an existing datapack that edits leaf loot tables, just to see how they did it. Anyways, good luck!