If it is due to the wing then I think is due to the way it opens.
The Ferrari one when it rotates back to close position it allows the moving element to rest on the main plane and so the wing aero load maintains it closed against the main plane backstop.
The RB wing when it rotates closed it rest against main plane center "rib" to prevent over rotation towards a classic drs wing position. Its unclear to me if the wing aero load maintains it closed or would try to rotate it back to the "parachute" position. I guess is down to where the pivot points are compare to where the wing load center of pressure is. It might be that the actuator is the one that keeps it close and not the aero load as the wing would be designed for a center of pressure that pulls the car down and therefor would want to rotate the wing towards the "parachute" position.

But the real problem with the RB design that I see is during cornering. The outside part of the wing will see extra load while the inside corner part will see less because of the end plate. In this moment the main plane center "rib" will act as a pivot point, the moving element will flex, and will open the inside corner gap and close the outside corner gap. I don't know is this decreases the down force on the inside or if it stalls the outside but the overall wing performance is reduces nevertheless.
Is not a how fast it closes is how stable it is in corner mode that is the problem. Max closed the wing a good second before he lost of the car so i don't see it as a reattachment issue.
If FIA changes the rules it should be to say that the "moving wings should rest on the main plane in close position" in which case the Ferrari macarena should be fine.