Summary
Only 1 glistering melon slice can generate in Ruined Portal chests. This is due to the loot table for ruined portal chests not specifying the minimum/maximum amount of glistering melon slices.

Fix:
Add this to glistering melon slice entry in behavior_packs/vanilla_1.16/loot_tables/chests/ruined_portal.json

  "functions": [
    {
      "function": "set_count",
      "count": {
        "min": 4,
        "max": 12
      }
    }
  ],