Reproduction steps:
1. Open project in "New Unity Project.zip"
2. Double click on "test" in the Project window
3. In Hierarchy of UI Builder Window select ScrollView
4. Note that in UI Builder Inspector ScrollView Mode is set to Horizontal, preview is visible in Viewport
5. Enter Play Mode

Expected result: Buttons are aligned horizontally
Actual result: Buttons are aligned vertically

Reproducible with: 2021.2.0a14
Not reproducible with: 1.0.0-preview.14 (2020.3.5f1)
Could not test with:
2018.4, because UI Builder is not available,
2019.4.25f1, because could not install com.unity.ui package (no version available for 2019.4.25f1)
2021.1.4f1, because of an Internal build system error (Backed exited with code 139), that would not allow entering Play Mode

Note: the issue seems to occur when the UI Builder package was merged into editor code