While there has consistently been issues with the trading engine (and especially websockets): rogue trades, orders stuck as "pending" for minutes, and most recently the two lock-ups (with websocks sending looping data in the second).
The DDoS defense is dynamic both with respect to the source and with respect to the destination.
So you are having the situation where only certain API URLs get protection but not the viewed-by-hand URLs (destination filtering).
Your experience will also depend on the current global DDoS botnet density estimate computed by IP address (source filtering).
To compund the above Prolexic doesn't disclose the details as a matter of policy. This is because they always have botmasters buying their service for their own machines to reverse engineer the Prolexic's defense strategy.
Was anyone getting HTTP error codes eg. 429 or 504? Or just Prolexic's standard timeouts due to dropped TCP/IP packets?