it keeps looping and resetting back to the default date.

Does `default date mean the date one month after the startOn date?

I'm a bit confused by the question and would like clarification. Based on your code, I assume the flow would then be:

API Response -> setPaymentData(data) -> User manually updates renew date -> Reset back to old value

I saw you checking the condition of data.results.paymentSummary.startOn && data.results.paymentSummary.renewOn, as well as data.results.paymentSummary.startOn individually, but did not see you check when data only has renewOn, are you missing that case or is it intentional?

Maybe adding case where data.results.paymentSummary.renewOn is not null and add logic there will solve the problem?

else if(data.results.paymentSummary.renewOn) {
  renewOn = localIsoString(data.results.paymentSummary.renewOn).slice(0,10); // Keep startOn the same
}

Also sorry for my two cent, but I think your code is mixing between snake_case and camelCase for property names which is not an ideal way to code JS, better keep it consistent by using camelCase, snake_case are mostly used for naming variable.

Your code flow