check "BKPF-Xreversal is NULL" in select statement

Hi ,

I have come across a wierd problem . I dont want the solution , I just want the reason why it happened.

In production environment below statement was working correctly .

select .....from BKPF into lt_bkpf  where Xreversal is NULL .

It gave correct result but after sometime it started failing.

I corrected this issue by adding below code.

   ( XREVERSAL = ' ' or XREVERSAL IS NULL ). Then it worked fine.

Can anyone tell me why the check which was working correctly before,  stopped working.

Since there was no change.

PS. Intially the first check was working in all the environment- ECD , ECQ and ECP. And it later failed in all the environment.