Portfolio USDC balance query returns 0 despite funds being present in the account.

I've developed code that performs transactions, and after a few days of trading the USDC holding value diminishes showing a 0 balance although there is a value in there, the only way I have resolved this is by moving the USDC to a new portfolio, redirect the CDP key to that Portfolio and then my code (unchanged) can then see the USDC balance again.

It's almost like after transactions the portfolio corrupts and damages the USDC wallet resulting in it not returning the value. Has anyone experienced this?