UniTaskSandbox1.cs

var hoge = new[] { 1, 2, 3 };
hoge.ToUniTaskAsyncEnumerable().ForEachAwaitAsync(async x =>
{
    Debug.Log($"Start hoge = {x}");
    await UniTask.Delay(TimeSpan.FromSeconds(1));
    Debug.Log($"Finish hoge = {x}");
}).Forget();

// Output
/*
0.0 | Start hoge = 1
1.0 | Finish hoge = 1
1.0 | Start hoge = 2
2.0 | Finish hoge = 2
2.0 | Start hoge = 3
3.0 | Finish hoge = 3
*/

UniTaskSandbox2.cs

var hp = new AsyncReactiveProperty<int>(10);
Debug.Log("Set hp = 10");

hp.ForEachAwaitAsync(async x =>
{
    Debug.Log($"Start hp = {x}");
    await UniTask.Delay(TimeSpan.FromSeconds(1));
    Debug.Log($"Finish hp = {x}");
}).Forget();

await UniTask.Delay(TimeSpan.FromSeconds(0.3));
Debug.Log("Set hp = 20");
hp.Value = 20;

await UniTask.Delay(TimeSpan.FromSeconds(0.8));
Debug.Log("Set hp = 30");
hp.Value = 30;

// Output
/*
0.0 | Set hp = 10
0.3 | Set hp = 20
0.3 | Start hp = 20
1.1 | Set hp = 30
1.3 | Finish hp = 20
*/

// 想像していた挙動1 / 全部流れてくる
/*
0.0 | Set hp = 10
0.0 | Start hp = 10
0.3 | Set hp = 20
1.0 | Finish hp = 10
1.0 | Start hp = 20
1.1 | Set hp = 30
2.0 | Finish hp = 20
2.0 | Start hp = 30
3.0 | Finish hp = 30
*/

// 想像していた挙動2 / ForEachAwaitAsync中のものは流れてこないが、最初の値は流れてくる
/*
0.0 | Set hp = 10
0.0 | Start hp = 10
0.3 | Set hp = 20
1.0 | Finish hp = 10
1.1 | Set hp = 30
1.1 | Start hp = 30
2.1 | Finish hp = 30
*/