When doing Item Availability Check in APO I need to use First date, what is the best approach?

When I do an ATP check for a sales order item in ECC, the material availability date is sent to APO. My requirement is to send the first date of the line item in the sales order instead of the material availability date.

I used exit 'EXIT_SAPLATPC_001' to change column 'bdter' of table 't_atpcsx' to achieve my result. I have managed to get first date through Stack.

Though it worked, I am having issues with BOP, as it deletes the schedule line when I run it the first time in APO after ATP check. Surprisingly, if I run it a second time, it fixes the issue and gives expected results.

I suspect an issue in BOP tables, but am not able to pinpoint it.

I would appreciate it if you could give me any input. Thanks.