Reproduction steps:
1. Open the attached 'AudioMixerPackage' project
2. Open the 'SampleScene'
3. Observe the 'My Source' GameObject in the Inspector
4. Click the small circle icon to the right of the 'Output' property in the Inspector to open an Object Picker window

Expected result: Audio Mixer Groups from the custom package folder are visible
Actual result: No Audio Mixer Groups are available

Reproducible with: 2019.4.35f1, 2020.3.25f1, 2021.2.13f1, 2022.1.0b9, 2022.2.0a5

Notes:
- Audio Mixer Group can be assigned by dragging and dropping it to the 'Output' property in the Inspector
- Audio Mixer Groups are available via Object Picker if they are located in the Assets folder