Why are my hazelnuts/filberts bitter?
Diarylheptanoids were recently identified as responsible for bitter off-flavor in hazelnuts. This correlates with


infection with bugs (of the bush, not in storage) and
starting germination 


according to: 
Singldinger: Molecular-sensory studies on the development of a bitter off-taste in hazelnuts, PhD thesis, 2019, Munich Technical University. 

Here's also a paper: Singldinger et al.: The Cyclic Diarylheptanoid Asadanin as the Main Contributor to the Bitter Off-Taste in Hazelnuts (Corylus avellana L.), J Agric Food Chem. 2017