Problem/Motivation
The EntityFieldManager's field map is not aware of bundle fields declared in code -- this is a longstanding core bug #3045509: EntityFieldManager::getFieldMap() doesn't show bundle fields.
Because of this, ContentEntityDenormalizer doesn't process any bundle fields, and if incoming JSONAPI data has a value for a bundle field, it will throw an exception.
Steps to reproduce
Proposed resolution
The field map is used rather than loading field definitions because it's a more lightweight data structure, so rather than switching wholesale, fall back to checking the field definitions when the field is not found.