There are a few possible reasons why only two of the dots disappeared while the third remained:

Visual Field Sensitivity: Your eyes and brain may have different sensitivities in different parts of your visual field. The bottom left area might not be as easily overridden by motion as the others. Each eye processes visual input differently, and slight imperfections or asymmetries in the brain's visual map could be responsible.

Eye Dominance: If one of your eyes is dominant (which is true for most people), it may influence how peripheral objects appear to "disappear." The brain favors input from the dominant eye, which could explain why the lower left dot doesn’t vanish as easily.

Peripheral Attention: Even though you're focusing on the center, your brain might be unconsciously more attentive to certain parts of your periphery, like the lower left, making it harder for the motion-induced blindness effect to kick in for that dot.

Micro-saccades: Your eyes constantly make tiny, involuntary movements (called micro-saccades) even when you're trying to fixate on a central point. If these tiny movements differ slightly in how they affect the lower left part of your vision, that could keep the dot visible.