Artificial Intelligence and Cognitive Science researchers have taken for granted that compositionality exists in the human brain.
Fodor , Chomsky, and many others have spoken about the human brain ability to create infinite meaning from a finite set of tokens (words).
There are people who argue that the brain is not compositional. You can read some of those arguments here: https://plato.stanford.edu/entries/compositionality/#ArguAgaiComp
I suspect the brain does have a compositional nature to it in that almost all learned concepts in the brain can be recombined with any other concept as a model building engine.
I will collect here empirical research that investigates compositionality in the brain.
A group of researchers examined people who watched movie clips while in an fMRI machine. The movie clips were specially constructed to have different variations of actors, objects, verbs, subjects, locations, and direction motions. The researchers then took the fMRI data and built classifiers for each of the individual variations and joint classifiers of the variations. Their classifier results show that the fMRI data in the brain does seem to represent those individual variations, in other words a neural representation of compositionality.