I have only been there 3 times, in September 2018, December 2020, and September 2022. I saw no Tiger Shark in 2018 trip. I saw 2 Tiger Sharks in 2020 trip, each in Submerged Rock & Manuelita Channel (big & pregnant mama Tiger Shark), as shown in the video, below.
I saw 3 Tiger Sharks in 2022 trip, 2 Tiger Sharks in Manuelita Channel twice, a Tiger Shark in Small Dos Amigos once, as shown in the video, below. The Tiger Sharks sighting in Manuelita Channel is shown around 7 minutes of the video and in Small Dos Amigos at 17 minutes: 20 seconds of the video.
Based on that limited data, I would say that you have a good chance to see a Tiger Shark in Manuelita Channel in September.
If you want 100% chance to see one, just go to Tiger Beach, Bahamas.