Read the SAP PowerBuilder 12.6 (http://help.sap.com/pb126) Installation Guide which mentions Windows 8.1 support
Installation Guide > Planning Your Installation > System Requirements
To justify for a potential bug which is not yet demonstrated, you have to narrow down the issue and share with us a use case example if possible.
Then we can check if it happens on the following environments:
PB 12.5.2 / Win7
PB 12.6 / Win8.1
Thanks for your understanding.
Jacob