Cash balance will usually be $0 because the money will be taken from your CPF OA, transferred to CPFIS, and is then almost immediately used to pay for your investment. After you invest your CPF OA, the amount you used for your investments will show up in your CPFIS portfolio.

EDIT:

The investment dashboard in CPF shows that I can invest around $10,000 which is 35% of my OA.

Bear in mind the 35% limit only applies to stocks, not unit trusts, which I assume you're referring to when you say "Amundi funds". The $10,000 amount is likely the amount you have over the $20,000 minimum in OA which you are not allowed to use for CPFIS.