(ensure 'auto-jump' feature is turned off)

Put this in a repeating command block:

execute as @a[nbt={OnGround:0b}] run say off ground

walk into a one-tall block (no jumping, no falling off cliffs).

EXPECTED: no output

ACTUAL: says "off ground"

Since this does not occur when walking against two-high walls, I think this might be the auto-jump logic somehow kicking in and affecting OnGround maybe? The player's Y coordinate never changes on the F3 screen, just the OnGround-edness for one tick when rubbing into a one-high wall.