Long block names such as "Smooth Red Sandstone Stairs" or "Polished Blackstone Pressure Plate" can overlap or even extend past the height info in the superflat customization screen. This is also an issue for many other languages which have longer block names compared to English.

To reproduce, use this superflat preset:
minecraft:smooth_red_sandstone_stairs,minecraft:waxed_weathered_cut_copper_stairs,minecraft:polished_blackstone_pressure_plate;minecraft:the_void

Related issues: MC-26757, MC-203757

A possible quick fix could be making the two columns more spaced apart, and anchroing them to the window border so more room is available when the window is larger.