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I put this block after `collection.set_norm(norm) in order to call the norm's getter.

# autoscale the norm with current accum values if it hasn't been set
        if norm is not None:
            if collection.norm.vmin is None and collection.norm.vmax is None:
                collection.norm.autoscale()

Seems to solve the issue.

To fix this issue, the code needs to be modified so that the string is converted to a proper normalization object before attempting to use it. The suggestion is to move the block of code that handles normalization ('norm') further down after it has been properly set.

This block checks and attempts to autoscale norm.
code-

    if norm is not None:
         if norm.vmin is None and norm.vmax is None:
               norm.autoscale(accum)

updated code :

def hexbin(x, y, norm=None, **kwargs):
    ...
    # Existing code for setting up the hexbin collection
    ...
    # Set the norm on the collection
    collection.set_norm(norm)

    # autoscale the norm with current accum values if it hasn't been set
    if norm is not None:
        if norm.vmin is None and norm.vmax is None:
            norm.autoscale(accum)
    
    # Continue with the rest of the hexbin logic
    ...
    return collection

This was already fixed by #28106.