In the attached project there are 5 Timeline Assets, and 2 menu item actions. One outputs the number of loaded Timeline Assets (via Resources.FindObjectsOfTypeAll<TimelineAsset>()), and another that unloads unused resources and tells the Garbage Collector to run.

Resources.UnloadUnusedAssets() returns a reference to an asynchronous coroutine. In the example code, the call to GC.Collect happens on the next line, so it's unlikely that this coroutine has had a chance to finish. I've done some quick modifications and get a final output of "In memory TimelineAsset count: 0".

using UnityEditor;
using UnityEngine;
using UnityEngine.Timeline;

public static class DisplayTimelineObjectCount
{
[MenuItem("Leak/Show Timeline Asset Count")]
private static void Display()
{
DebugOutput();
}

[MenuItem("Leak/Unload And GC")]
private static void UnloadAndGC()
{
AsyncOperation operation = Resources.UnloadUnusedAssets();
operation.completed += (_) =>
{
System.GC.Collect();
EditorApplication.delayCall += DebugOutput;
};
}

static void DebugOutput()
{
var assets = Resources.FindObjectsOfTypeAll<TimelineAsset>();
Debug.LogWarning($"In memory TimelineAsset count: {assets.Length}");
foreach (var ta in assets)
{
Debug.Log($"Found Asset : {ta.name}");
}
}
}

If, instead of immediately trying to invoke the GC , you wait for the UnloadUnusedAssets coroutine to complete, you should see all your assets get collected.