Focus shift on the EF 50mm f/1.2L is generally a characteristic of its optical design, not random copy-to-copy behavior or a manufacturing defect. It comes from residual spherical aberration, which can cause the plane of sharpest focus to move slightly as you stop down.

In practice, though, the effect is usually subtle. At around 3 feet, you may simply be far enough away that it’s hard to notice in real-world shooting. It tends to be more apparent in controlled tests, with high-contrast targets and very precise focus evaluation, and can become more noticeable at very close distances—especially when extension tubes are used to focus closer than the lens’s normal minimum focus distance.

With subjects like flowers, small movement from breeze or normal subject detail can easily mask a slight focus shift. So if you’re not seeing it, that’s not unusual: the lens can still exhibit focus shift in theory, but under your shooting conditions it may be too minor to matter.