I am not sure whether it would be possible to fix, since backface culling is calculated from the player's location; so F5 mode makes sense that it would not work correctly.
Water is rendering incorrectly in F5 mode, with extra water block faces being rendered when they shouldn't.
The second screenshot is showing the faces of the water behind the flowing water because the player character itself is past that block boundary, so in first person those faces should be exposed as they are.
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Affects 19w02a and 19w03a

Affects 19w03b

Affects 19w04a

If someone with the required permission would like to, I would be alright with transferring ownership to Connor Steppie.
This bug will likely be fixed once the transition to Blaze3D graphics library is complete, but it is still useful to keep track of problems that need to be tested once the library is released.

This seems to have been fixed in 19w11a.
Can confirm for 18w50a, can I request ownership of the ticket?