The bug

When a loot entry uses the tag type and has expand set to true, any functions on that loot entry won't be applied to the item.

How to reproduce

1. Add the loot table shown below to a data pack or download the attached data pack MC-212671.zip.
2. Use the /loot command to give yourself the loot table
3. Notice that you only get one item, when you expect to get a full stack.

Example loot table

{
  "pools": [
    {
      "rolls": 1,
      "entries": [
        {
          "type": "minecraft:tag",
          "name": "minecraft:logs",
          "expand": true,
          "functions": [
            {
              "function": "minecraft:set_count",
              "count": 64
            }
          ]
        }
      ]
    }
  ]
}