Diagnosis
This is called "drift," and it's caused by a very slow leak in the hydraulic system inside the lifting columns. It's a known issue with some electric sit-stand desks, including Branch models, over time.
How to Fix It
There's no user-serviceable fix for the internal seals. The first step is to contact Branch support. They have a warranty (typically 5 years on the frame/motor) and this is a classic warranty claim. They'll likely ask for a video showing the drift over a set period. Be prepared to provide your order number and a photo of the serial number sticker on the underside of the desk.
If That Didn't Work
If you're out of warranty, the permanent fix is replacing the affected lifting column (the leg with the motor). Branch can sell you a replacement column. It's a big job involving disassembling the frame, but it's cheaper than a whole new desk.
Heads Up
Don't try to stop the drift by putting something under the desk to block it. You'll confuse the anti-collision system and potentially cause an error or motor strain. Just let it drift and get the process started with support.