When chunk borders are enabled (F3 + G) and you go underwater, the vertical lines of the grid disappear.
What they actually do is turn to a shade of blue almost exactly like the water, but it's only noticeable when you get really close, therefore basically disappearing
Examples below:
Top: Underwater (Bugged) behavior
Bottom: Above-Water (Expected) behavior
Cannot reproduce in 1.16.4-pre1