Search on many2many with negative operator gives wrong results

When you have a category X and assigned it to one or more partners, it is currently impossible to search for partners that do not have this category.

Steps to reproduce
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On a new database,

Create partner A
Create partner B
Create partner C
Create partner AB
Create category A
Create category B
Assign category A to partner A
Assign category B to partner B
Assign both categories A and B to partner AB
Do not assign any categories to partner C

Now search for partner with advanced filter on category 'not equal to' A

Expected results
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All partners that are not assigned category A (i.e. partners B and C)

Actual results
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All partners that are assigned any category other than A, including partners that also have category A (i.e. partners B and AB)