DIAGNOSIS This is almost always a loose or failing connection inside the cable or at the port. The jostling disrupts the signal. Yeah, this one's common with any portable monitor that gets moved around a lot.
FIX Firmly reseat both ends of your video cable (USB-C or HDMI). If it's USB-C, try flipping the connector 180 degrees. The primary fix is to replace the cable. Use a high-quality, short, reinforced USB-C cable rated for video data.
IF THAT DIDN'T WORK The issue could be a slightly loose internal connection where the port is soldered to the monitor's board. If a brand-new, known-good cable still causes flicker with movement, that points to a hardware fault requiring warranty service.
HEADS UP Avoid putting any strain on the cable where it plugs into the monitor. Consider a right-angle USB-C adapter if you need to route the cable in a tight space to prevent constant wiggle.