I've been successful in provisioning a Page using an extensibility handler when executing through a console application. I'm using the PnP/Samples/Provisioning.Extensibility sample code and it's working as expected.
I then take the Provider section of the provisioning template and stick it into a .PnP template file and use the ParnerPack to provision a sub-site using the template.
I can step through the extensibility handler code as it's called, and it seems to do exactly the same as when called from the console app... no exceptions are raised... and yet - the page is never created!
What could be the cause of this failure? The only difference (obvious to me) is the credentials with which the process is executed..the PartnerPack is running with App-only permissions, using a certificate to authenticate. The console app was running with my credentials (site collection owner).