What’s causing rear derailleur wobble on largest cassette ring?

I just finished installing and setting up this bike – it's a Cannondale CAAD4 frame.

Parts:

• Shimano 105 RD-R7000 GS 11-speed rear derailleur
• Shimano 11-speed cassette 11-34t
• Shimano 105 50-34 chain rings
• Shimano chain set to length of 108 links per calculations here

The shifting on the rear derailleur is smooth and steady until I shift to the largest ring on the cassette, then the derailleur basically gets a "wobble" to it.

https://reddit.com/link/1nz6i0d/video/owf3tv16cetf1/player

What's causing this? According to Shimano the RD-R7000 GS derailleur should be compatible with the 11-34 cassette. This occurs when the front shifter is on the smaller chainring (or the larger, but I wouldn't ride like that).