nimbleFunction constructs and evals a reference class definition. When nimbleFunction is used in package source code, this can lead to problems finding things due to namespace issues. Using where = getLoadingNamespace() in a nimbleFunction in package source code should solve this problem.
getLoadingNamespace()nimbleFunctions defined in the NIMBLE source code use where = getLoadingNamespace(). Please let the NIMBLE developers know if you encounter problems with this.