I redid the objective and it seemed to clear things up, so at least for me the problem went away, though I do believe I went through the synthesis tutorial beforehand, so this would be me doing it a second time. I wonder if doing it as part of the new strange doesn't count somehow? even if it's the same tutorial, it might be treated differently.
I also have only just come back to warframe after a few months of not being on, and I did the tutorial during the time I played before now, so that might have triggered it.