How to reproduce:
1. Create a new Universal Render Pipeline project
2. Open the Package Manager
3. Download and install the ProBuilder package
4. Import into Project the ProBuilder package LWRP Support sample
5. Open the Assets/Samples/ProBuilder/4.1.0/LWRP Support/Material folder
6. See the ProBuilder Default LWRP.mat

Expected result: material is displayed correctly
Actual result: material is displayed pink; reimporting it fixes this issue

Reproducible with: 2019.3.0a10, 2019.3.0b6, 2020.1.0a8
Could not test with: 2017.4.33f1(ProBuilder package unavailable), 2018.4.11f1, 2019.2.8f1, 2019.3.0a9(ProBuilder Samples unavailable)

Reproducible with Universal RP package versions: 6.5.2, 7.1.2
Could not test with Universal RP package versions: 5.16.0(Incompatible with reproducible Unity versions)

Reproducible with ProBuilder package versions: 4.1.0
Could not test with ProBuilder package versions: 4.0.5(Samples section unavailable)

Notes:
Reimporting the sample material displays it properly
Does not reproduce with HDRP