Secret token disappeared after first success build?

## Issue

### Context
- **Gitlab plugin version**: 1.5.11
- **Gitlab version**: 11.9.11
- **Jenkins version**: 2.150.3
- **Job type**: Pipeline, Freestyle, Matrix, etc.

### Logs & Traces
N/A

### Problem description
I integrated jenkins trigger with gitlab webhook. I can be triggered 200 successfully with secret token. But after the first hook, secret token disappeared on Jenkins side. When gitlab try to post another one, it could not reach because the same token not exists on Jenkins side. Is this bug or am I missing something ?
I can success with disable authentication for '/project' end-point.But for security I don't want to disable it.