If you read C:\AVEVA\Plant\PDMS12.0.SP6\Manuals\Docs\PDMS User Bulletin12.0.pdf Chapter 23.7 Incompatible PML changes, you will see that some in house customisation has to be checked and modified to be 12 compliant.
HTH
There have been multiple changes on some of the pml used in 12.0 that changes the language from the 11.6 scripting. Best thing to do is a compare/merge of all of your development against the out of the box versions in 11.6 and 12.0.