Cross-references fail to resolve on big `xrefmap.xml` file

**Operating System**: `macOS Big Sur 11.6 (20G165)`
**DocFX Version Used**: `2.59.2.0` (brewed)
**Template used**: `default`

**Steps to Reproduce**:

Attached there's a Unity project with two configs: `docfx_tiny.json` and `docfx_complete.json`

[unity-test.zip](https://github.com/dotnet/docfx/files/8904240/unity-test.zip)

1. Download unity-test.zip and unzip.
2. Run `docfx build docfx_project/docfx_tiny.json --force`. See how it resolves a valid `StartCoroutine_Auto` reference.
3. Run `docfx build docfx_project/docfx_complete.json --force`. It doesn't resolve a valid `StartCoroutine_Auto` reference.

Both xrefmap files define this entry:

```
- uid: UnityEngine.MonoBehaviour.StartCoroutine_Auto(System.Collections.IEnumerator)
name: StartCoroutine_Auto
href: https://docs.unity3d.com/Documentation/ScriptReference/MonoBehaviour.StartCoroutine_Auto.html
commentId: M:UnityEngine.MonoBehaviour.StartCoroutine_Auto(System.Collections.IEnumerator)
fullName: UnityEngine.MonoBehaviour.StartCoroutine_Auto
nameWithType: MonoBehaviour.StartCoroutine_Auto(IEnumerator)
```

**Expected Behavior**:

DocFX resolves big `xrefmap.xml` files (sample file attached is ~14mb).

**Actual Behavior**:

xrefmap with one entry:

big xrefmap (containing the entry present in the previous xrefmap, but not resolved):