How to reproduce:
1. Open the project "case_1234614-WebGL_transform.position"
2. Open the scene "SampleScene"
3. Make a WebGL build and run it
4. Observe the results in the Console window

Expected results: the values of the Transform.position of the GameObject "Image" are the same in the Start() and in the Update() methods
Actual results: the values of the Transform.position of the GameObject "Image" are different in the Start() and in the Update() methods

Reproducible with: 2019.3.0a11, 2019.3.8f1, 2020.1.0b5, 2020.2.0a7
Not reproducible with: 2018.4.21f1, 2019.3.0a10

Could not test with: 2017.4.39f1 due to errors

Tested on: Chrome (81.0.4044.92), Firefox (75.0), Microsoft Edge (44.18362.449.0)

Notes:
1. It is only reproducible when the Canvas Scaler is set to Scale with Screen Size
2. It is not reproducible with the Transform.localPosition
3. The values of Transform.position changes after the first execution of the Update method
4. It is reproducible only on WebGL builds