How to reproduce:
1. Open the project "case_1225427-ProBuilder_UVtemplate"
2. Open the Scene "SampleScene"
3. Select the GameObject "Cube" in the Hierarchy
4. Press "Open ProBuilder" button in the ProBuilder MeshFilter component in the Inspector
5. Select UV Editor in the ProBuilder window
6. Select Face Selection in the UV Editor toolbar
7. Select any face in the UV Editor window
8. Select Render UVs in the UV Editor toolbar
9. Select Save UV Template in the Render UVs panel
10. Save the file in the Assets folder
11. Open the UV Template file from the Project window and observe the results

Expected results: the top UV line is missing in the rendered UV Template
Actual results: all UV lines are in the rendered UV Template

Reproducible with: 4.0.2 (2019.3.6f1, 2020.1.0b2, 2020.2.0a3), preview.4 - 4.3.0 (2018.4.19f1, 2019.3.6f1, 2020.1.0b2, 2020.2.0a3)
Not reproducible with: 3.1.10 (2018.4.19f1)

Could not test with: 2017.4.37f1 because the ProBuilder package is not available, 4.0.2 (2018.4.19f1) due to Console errors

Notes:
1. The UV lines become broken in the UV Template when the selected Image Size is 2048 or 4096 in the Render UVs panel