Repro steps:
1) Open the project and only scene in it
2) Try writing something in the InputField and notice in the console that the index increments properly
3) Click "Copy to InputField" button and notice that the index is always 0 (or nothing happens in 5.3 and 5.2)

Expected: every letter increments properly when setting the InputField text property

Reproduced in: 5.4.0b8(charIndex is always 0), 5.3.3p2; 5.2.3p3 (OnValidate not even registered - empty console on button click)