After some time, the searches of a particular type becomes unreliable, and searches incorrect.
These are searches on calculated fields that include related fields from multiple tables (eg. Table A relate to Table B)
After restarting the FileMaker Server 12 searches are normal again.
We see this error in 2 independent files on the same FileMaker Server 12.
In FileMaker 11 we did not have this problem, the searches were slow (unindex), but the result was correct.