How to reproduce:
1. Open attached project (1226349.zip)
2. Enter Play Mode
3. In the Game window, there should be an Input Field, select it and type "test" in it
4. Select the text and press Ctrl+Insert (to copy)
5. Deselect the text and press Shift+Insert (to paste)

Expected result: The "test" text was copied and pasted into the Input Field and now its text is "testtest"
Actual result: Nothing changed and the text in the Input Field remained "test"

Reproducible with: 2018.2.0a4, 2018.4.19f1, 2019.3.6f1 (1.0.0), 2020.1.0b2 (1.0.0), 2020.2.0a3 (1.0.0)
Not reproducible with: 2017.4.38f1, 2018.2.0a3

Notes:
1. The Ctrl+Insert (copy) and Shift+Insert (paste) shortcuts work in the Editor, but not in the in-game "Unity UI"
2. Couldn't test on a Mac since its keyboard doesn't have the Insert key