In 18.50 the only memories of Cynthia on Day 3 Afternoon are unlocked after telling her Roger is the one intern happy. This can be achieved at the end of the section 'Hospital - Day 3'. Maybe you can try to replay this section.Finally got it: it requires putting all 4 girls from the demonstration in the hospital to trigger the modified Rosario event in the afternoon. I guess that means the parasite is in one of those 4 and I look forward to finding out which.
I have one last memory to unlock in v18.50: Cynthia, Day 3 Afternoon. This is related to the injured witness information but I already got that a long time ago and gave it to Cooper as previously, that was the only way to get to day 3. As I recall, I used a stolen dress from the Theater to get this but I'm missing the "you can use the explosion as a distraction" memory.
Looking at the walkthrough, it really feels like this is something I should have unlocked already ... Either way, I can't seem to make Cynthia ask me for a distraction again.
This is not really related to the injured witness, which is the first quest of Cynthia (day 2 - day 3). The happy intern is the second one (day3 - day 4).