Issue 945: Rejoining server causes prefabs to become unregistered

Repository: miragenet/mirage

Issue body:
**Describe the bug**
Joining a server, leaving, and then joining again causes prefabs to get unregistered and they can no longer be spawned, resulting in the client being thrown out of the server instantly. 

**Repro project**
[Mirage Issue.zip](https://github.com/MirageNet/Mirage/files/7219730/Mirage.Issue.zip)

**To Reproduce**
Steps to reproduce the behavior:
1. Open the attached project
2. Go into the SampleScene in the Scenes folder.
3. Make a build and run it.
4. Create a server in either Unity or the built client.
5. Join the server.
6. Leave the server
7. Join the server again
8. Observe the issue

**Expected behavior**
The user should be able to rejoin the server.

**Desktop (please complete the following information):**
* OS: Windows 10 64-bit
* Build target: Windows 64-bit
* Unity version: 2020.3.17
* Mirage version: 105.1.0
