I'm not surprised as I have seen a few other postings over the past year where a PB 12.6 issue was also reproducible in 12.5.2 but not in an earlier version to that. I suspect that SAP was making more changes in 12.5.2 than just removing the WebForms feature. 
Either way, it sounds like a ticket needs to be opened up for this issue by SAP Technical Support for engineering to address this defect.