TRUE ≠ 1 and odd behaviour after import

LS,

Maybe this is well known, but couldn't find post about it.

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So a client of mine had a database crash. I recovered the database ( 7GB ) and imported the data into a clone of a backup.
There is a relationship of any  to multiple between two tables, eg. link_all (  value : 1 ) and a field called active_link with this calculation 

Status= "On Hold" or Status="Active" or Status="Booking" ( result : number ) ( this is a boolean result )

For some strange reason the imported data continued to behave like they always did, eg. the result of this calculation was 1. ( field value : Active )

All records created after the import had no value  ( field value : Active ).

The fix was easy : 

case (  Status= "On Hold" or Status="Active" or Status="Booking" ; 1 )

The question remains  : why is FM changing is behaviour all of a certain with only new records ? 

Kind regards,

Joost