I had the same problem with Chapter 4 -- "Plant" objective was 128/141 and I could not find Dr. Styles to complete the dialogue with him.
Discovered from previous save that when I talked to Dr. Styles, my "water" objective was only 41/43. Therefore, my conversation with Dr. Styles did not complete.
I went back to a save early in the chapter and played it over. It went very fast by skipping dialogue and knowing where to go etc.
Make sure your "Water" objective is 100% before talking to Dr. Styles.
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