How to reproduce:
1. Open attached project "Case_1201105"
2. Load "SampleScene"
3. Make sure you have Japanese IME installed
4. Enter Play Mode
5. Start rapidly typing "a" (あ using Japanese IME) and backspace
6. Enter and delete the character ~50 times

Expected result: you can type and delete Japanese characters with no errors
Actual result: quickly typing and deleting Japanese characters throws an exception:
"ArgumentOutOfRangeException: Index and count must refer to a location within the string.
Parameter name: count"

Reproducible with: 2017.4.35f1, 2020.1.a15

Notes: Bug occurs very rarely, between 3 and 200 cycles of entering and deleting a character with the average being ~40-50. Significantly easier to reproduce using Unity Editor on iOS due to faster working IME (in order to reproduce more consistently on Windows, Build and Run the project). Reproducible using both default Input Field and the TextMeshPro Input Field.