What do you mean by create new package definitions? Having this problem with a recently created pkg from Jamf Composer.
I only meant when you have a new package to deploy, create a new entry for it in the Jamf->Computer Management -> Packages section, instead of updating an existing package definition entry there. That seems to solve the recent issue of invalid checksums (an issue Jamf Admin used to handle for us!). After you create your new entry, it is OK to update it afterwards (seems to be a 11.9 glitch with the removal of a "Ignore" setting in the verify packages sections of Computer Management -> Security settings).