Thanks for correcting me, EJ22B.
I read about the unplanned rear wing change in FP1, but without checking had assumed that they just replaced it with another example of the same spec.
Andrew Hooper (@WilliamsSupport), who was a VIP guest at Williams on Friday, said it didn't break, but there appeared to be issues with its structural stability, forcing them to take it off. As far as I could find, Williams themselves didn't mention the unsuccessful rear wing test at all.
I tried to find some good images of the new beam wing, which was part of the upgrade, but that turned out to be difficult. Even though the rules were clarified so that the show-and-display car is supposed to feature all new parts now, Williams showed off Sargeant's car without them. And during FP1, they didn't run the wing for long.
Here's one decent image of the new beam wing I could find.
It's quite a departure after staying with variations of the same basic design since the launch of the FW44.