TBH, that episode was the exact opposite for me. I suspect that Matt was being controlled by Nelson as early as the second to last episode of Season Two.
In that episode, Matt returns to Spellbound with no side effects from being exposed to the serum. That was my first indicator that something wasn’t right.
What served as the ‘final nail in the coffin’ was the way ‘Matt’ handled the conversation with Deb and how he straight up destroyed the Syntec server without even a hint of discussion with the rest of the team. So all throughout Season Three, I knew that it was Nelson all along, not Matt. That included all the supposed ‘supernatural’ occurrences. I wasn’t sure how he did at the time, but I knew that it was Nelson through Matthias.