why system is not allowing to update firmstartdate on planned order using Rest API?

Summary:

System is not updating firmstartdate on planned order the even though rest API response is 200OK

Content (please ensure you mask any confidential information):

we are unable to update firmstartdate on a planned order as we want to start date of the order to be specific to the date we set.(calculated apex).
but system not updating the same even though API response is 200OK.
what is the issue here?

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