March 24th, 2021 - replicated the issue in our project and a blank project. Same assertation on the same line. Using Unreal Engine 4.26
Still unable to successfully package DLC.
Assertation seems to trigger during the BlueprintCodeGen.
The line the assertation points to is checking the validity of the Plugin ptr shown below.
Tsharedptr Plugin = IPluginManager::Get().FindPlugin(cookbythebookOptions->DLCName)