How to reproduce:
1. Open the attached project "1261788.zip"
2. Open the "Test" Scene
3. Select "Canvas" GameObject in the Hierarchy window
4. Check/uncheck the "Pixel Perfect" checkbox of Canvas Component

Expected result: text sharpness changes when checking/unchecking the "Pixel Perfect" checkbox
Actual result: text sharpness does not change

Reproducible with: 2018.4.25f1, 2019.4.4f1, 2020.1.0b16, 2020.2.0a18

Notes:
- The sharpness of all text updates according to the value of the "Pixel Perfect" parameter when:
- - Disabling and re-enabling Canvas Scaler component of "Canvas" GameObject or the "Canvas" GameObject itself
- - Entering the Play Mode
- - Switching UI Scale Mode of Canvas Scaler component of the "Canvas" GameObject to Constant Physical Size
- - Changing Reference Resolution Y or Reference Pixels Per Unit parameter values of the Canvas Scaler component of the "Canvas" GameObject
- Disabling and re-enabling the Text component of the "Text" GameObject or the "Text" GameObject itself only updates the text sharpness of that GameObject
- Changing any parameters of Text or Rect Transform components of the "Text" GameObject only updates the text sharpness of that GameObject