If a Folder/Project has a Default Workflow (for tasks) set, but is also tagged in another location where a Custom Workflow is applied, the Custom Workflow will indeed override the Default one.
I.E. tagging a folder or project (a) which has the default workflow set to a folder (b) which has a custom workflow defined will automatically change the default workflow for folder project (a).
This does not occur if project (a) has any custom workflow assigned.
In other words: it is not possible to assign the default workflow to a project or folder (a) if any parent (b) folder has a custom default workflow defined.
The default workflow for (a) can be selected using the change-workflow function, but this will not take any effect.
The default workflow should be treated like any custom workflow.