I'm sorry about that. Let's look into it.
Could you enable logging for the time being? WP Admin > s2Member > Log Files
Do these users have an EOT time set in their profiles?
You see, trials are part of a subscription, and that's managed by the payment gateway, not s2Member. The gateway notifies s2 about payments or cancellations.
Or how did you set up your membership trial? Please give me more details about your setup.
I look forward to your update.