How to stop booking number from changing once the automation is run

[Aditya_K1999]

I have a table by the name of Service booking, which has a booking number field with the below formula:

(CONCATENATE("MAC"," - ",LEFT(TEXT([Booking Date]),1),MID(TEXT([Booking Date]),6,4)," - ",UPPER(RIGHT([Booking ID],2))))

I am running an automation on the table when the "Generate Estimate" Button is clicked on the table. The purpose of the automation is to generate an estimate file.

But after the automation is run, it is also changing the booking number...how to stop that?