How to reproduce:
1. Open the attached project "case_1313831 RectTransformUtility_Not_Working.zip"
2. Open the Scene "SampleScene"
3. Select the "Canvas" GameObject from the Hierarchy window
4. In the Inspector window, make sure that Render Mode is set to "Screen Space - Camera" and Plane Distance is set to 0.3
5. Select the Console window and enter Play mode
6. Move the mouse around and observe the values output to the Console window

Expected result: Mouse Position in the world-space and on the Canvas relative to the Position on-screen is returned
Actual result: (0, 0, 0) and (0, 0) are returned constantly

Reproducible with: 2018.4.32f1, 2019.4.21f1, 2020.2.7f1, 2021.1.0b9, 2021.2.0a7

Notes:
- Increasing the Plane Distance just by a little bit, for example to 0.3001 instead of 0.3, resolves the issue
- RectTransformUtility.ScreenPointToWorldPointInRectangle() and RectTransformUtility.ScreenPointToLocalPointInRectangle() are the functions with the issue, other functions work as expected