There is an issue when importing FBX that creates new materials when using the search materials import setting. Instead of being able to use the material that is in the Base Material name, the texture and material imports anyway. This issue only occurs if the material being imported contains a texture. This issue is a regression.
4.16.3 - CL: 3561208 - Issue does not occur
4.17.2 - CL: 3658906 - Issue occurs
4.18.0 - CL: 3709383 - Issue occurs
Head over to the existingAnswerHub thread and let us know what's up.