It is recommended to check the VALID bit always and clear it as recommended in AN219842. It is the proper way to use the fault structure.
Following are some instances when VALID bit can be "0", but ISR gets triggered:
2a. There are faults during system startup before OS initialization when the ISR is disabled. So, fault ISR is pending.
2b. The fault registers are cleared.
2c. ISR initialization and enabling happens after this.
2d. The pending ISR will get triggered. But, the fault register VALID bit will be "0".