Steps to reproduce:

1. Open an existing BiRP project containing terrain details (grass). Alternatively, download the repro project (link in the comments)
2. Upgrade the project to URP
3. Go to Edit > Rendering > Materials
4. Convert all project materials
5. Notice that terrain details do not get automatically converted

Notes:

- There's a workaround to this: after the failed conversion, select the terrain > Paint Details > select affected objects > Edit Details > Edit. Affected materials will be converted upon executing those steps
- Observed in 2021.2.0a10 using URP 12.0.0 on macOS