The bug
The effect for entities swimming in water ignores the height that the water actually is. This means that if water exists in the block space in any form, any entity that enters that block space, at the height of water level 5, will be pushed by flowing water (via slabs, stairs, lanterns, stonecutters, jumping, etc.), and be capable of entering the swimming mode, although the height is corrected after players begin to swim.
Video Example (with mobs):
Linked issues
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Comments

Could you add any examples or information as to how this affects gameplay?

It appears that the currently invalid details for lava in 1.16.0.59 on MCPE-74990 are the same for water (confirmed for 1.16.0.66 at least). For example: when setting up the device shown in the first image on MCPE-74990, but instead swapping lava with water, mobs don't swim and will drown on spot.
[media]If the same device were edited to be used on players, they will be able to move at normal walking speed, riptide tridents won't work, they'll experience MCPE-59493 (but in water of course), and they'll drown eventually. It's hard to make videos right now, so I'll include an image for villagers:

Affects 1.16.100
Affects 1.16.200.56 Beta

Affects 1.17.40.20

The latest beta is already listed in the Affects Versions list.
Relates to MCPE-45633.