Reproduction steps:
1. Open the attached project "1422293_19.4.zip"
2. Open the Package Manager window
3. Select one of the "Custom" packages
4. Click "View documentation"
Expected result: Package Manager shows the offline documentation files
Actual result: A "This package does not contain offline documentation." message pops up
Reproducible with: 2019.4.35f1, 2019.4.38f1, 2020.3.25f1, 2020.3.34f1
Not reproducible with: 2019.4.34f1, 2020.3.24f1, 2021.3.1f1, 2022.1.0f1, 2022.2.0a11
Notes:
1. Tested on macOS
2. Similar issue: case 1397413