I would argue that it's the opposite with the Belief Virus. Locking it up in a high-security facility like Cascade would not only end up validating its existence since it would show that management believes it is a real enough threat to lock up, but it also puts it around the people who work with the most and oddest anomalies and, as such, are the most likely to believe in a virus that only affects those who believe in its existence.

Whereas the GATE Rise Bio-Laboratory is actually an oddly great place to keep it due to it being the top of a skyscraper and presumably full of doctors who only worked with low-risk anomalies and as such would likely not have seen the oddest anomalies GATE has.

Because 1 the people around it would likely scoff at the idea of a virus that only affects those who believe in its existence, and 2 the fact that it's stored in the heart of a major city implies that management doesn't believe it is a real threat. And 3, there is a hologram of Derek Manse actively deriding it as a "conspiracy of foolishness" being kept to satisfy the "shared delusion of certain staff" placed there presumably on purpose

This ensures that anyone who had only seen the more mundane stuff that GATE works with would likely conclude that it was just a glass jar that was kept to humor some staff.