Reproduction steps:

1. Add the attached shader "Vulkan_Metal_missing_struct_reflection.shader" to a project
2. Select the shader
3. In the shader's Inspector Window select the dropdown next to "Compile and show code" and set "Custom" and "Vulkan" or Metal
4. Click the "Compile and show code" button

Expected result:
Shader compiles for Vulkan or Metal without errors

Actual result:
Error message "Struct 'myStruct' is not found." is printed to the Console Window and only "Vector4 myStruct.m_Float4 at 0" is present in the "Constant Buffer" reflection part of the compiled output (no struct definition)

Reproducible in: 2021.2.0a7, 2021.1.0b9, 2020.2.6f1, 2020.1.17f1, 2019.4.21f1, 2018.4.32f1