How to reproduce:
1. Open user-submitted project (MeshCombineTest.zip)
2. Open the SampleScene
3. Enter Play Mode

Expected result: no assertions are thrown in the Console window
Actual result: two assertions are thrown in the Console window

Reproducible with: 2019.3.0a11, 2019.4.9f1, 2020.1.4f1, 2020.2.0b2
Could not test with: 2018.4.26f1, 2019.3.0a10(‘Mesh’ does not contain a definition for ‘GetVertexAttribute’)

Notes:
The following assertions are thrown:

Assertion failed on expression: 'uvChannel.dimension == 0 || uvChannel.format == kVertexFormatFloat'
UnityEngine.Mesh:CombineMeshes (UnityEngine.CombineInstance[])
MeshMergeTest:Start () (at Assets/Scripts/MeshMergeTest.cs:27)

Assertion failed on expression: 'chanType.dimension == 0'
UnityEngine.Mesh:CombineMeshes (UnityEngine.CombineInstance[])
MeshMergeTest:Start () (at Assets/Scripts/MeshMergeTest.cs:27)