System Information
Operating system: Windows 10 version 10.0.19041
Graphics card: NVIDIA GeForce RTX 2080

Blender Version
2.83.2

Short description of error
When use a driver in a particle system the particle system doesn't react to driver value change

Exact steps for others to reproduce the error

1. Create a particle system emitter
2. Create a trigger object
3. Add a custom property to the trigger object
4. Relate the property with Velocity.Normal (var = Cube.["trigger"]) Expression: var*10
5. Run animation
6. Change the trigger property value to 0
7. Velocity.Normal changed to 0 but particles are still emitted

Look at the attached example.