No budget availability control for negative budget

Kindly I need your help regarding a strange issue which I am facing it.

It is related to SAP-FM The issue is I have uploaded the budget for FY-2023 CB & as well PB. I have CI with a budget amount of 2,000 USD as a total with different FC. The user creates a purchase request using the account assignment cost center, this cost center is mapped with FC which does not have any budget amounts on it. the system has saved the PR and in the budget report consumption, it’s showing the figures with a negative sign. How the system accepted this transaction with no budget on that FC? And how AVC in FM-module did not check the budget?

Thanks