In a heated battle for third place in the latter stages of Saturday's first of two races at the ExCeL, defending series champion de Vries was attempting to keep a hard-charging Cassidy at bay and fought at close quarters until the end.
Despite his Envision rival's best efforts, Mercedes driver de Vries was able to clinch third at the line - and an incident investigated where de Vries was alleged to have moved under braking was given no further action.
But the FIA has reviewed another of de Vries' defensive moves, and felt that in the penultimate lap, the Dutchman moved twice in trying to hold off Cassidy into Turn 1.
The official FIA steward's document reads: "In the pre-ultimate lap the driver of car 17 [de Vries] moved more than one time in front of car 37 [Cassidy] on the Start and Finish straight before T1 to defend his position."
A five-second penalty thus drops de Vries to sixth as Cassidy assumes third place ahead of Oliver Askew and Mitch Evans.
Asked if he felt that de Vries' moves were borderline, Cassidy said: "We got there and got to fight for the podium, but yeah, sounds like you've got the same feeling as me.
"I'm just disappointed myself for qualifying [seventh], because I feel I've been putting laps together a lot recently and and the group was okay.
"I just clipped the wall in the duel and so I feel probably I could have had a good fight with Nyck in that duel and probably should have started top four. That would have made the race a bit different."
Evans' move up the order cuts his championship arrears to Stoffel Vandoorne from 26 to 24 points, while Askew improves on what was already his best finish of the season - his second points finish of the year - in fourth.
Porsche has had a penalty rescinded after being alleged to have used electromagnetic radiation between 5 and 6GHz, having originally received a fine - but a further stewards' report cleared it of doing so.
The FIA stated on the initial document: "It is forbidden to use an electromagnetic radiation between 5 and 6 GHz because it can disturb the data system of FIA.
"In case of continuing this usage the Stewards may impose a more severe penalty up to a disqualification."
