Steps to reproduce:
1. Open the attached project (869386-TestProject.zip)
2. Open the Assets/bug_report_anim_shader.unity scene
3. Play the scene
4. Compare the left (Render Mode = Billboard) and right (Render mode = Mesh) samples (They both have Anim shader)

Expected result: Particles Anim shader should work on mesh particle mode
Actual result: Particles Anim shader does not work on mesh particle mode

Note:
With 5.6.0b3 Particles Anim Shader does not work on billboard particle mode
Shader introduced in 5.5

Reproduced with: 5.5.0a1, 5.5.0f3, 5.5.0p4, 5.6.0b3