The fluid minecraft:FLUID (minecraft:water and minecraft:lava) is only used for source blocks, non-source blocks are minecraft:flowing_FLUID. (minecraft:flowing_water and minecraft:flowing_lava)

1. FLUID[falling=true]
The fluid minecraft:FLUID has the fluid state falling=true|false.
The falling fluid state is supposed to indicate if it's a fluid flowing downward from a source block, however, this means it's always false for source blocks itself as it can exist on it's own, not with help of another source block.

2. flowing_FLUID[falling=false,level=8] & flowing_FLUID[falling=true,level=1-7]
The fluid minecraft:flowing_FLUID has the fluid states falling=true|false and level=1-8.
when falling is set to true, level is always set to 8, and level is never set to 8 when falling is false.

The fixes:
1. simply remove the falling=true|false fluid states from minecraft:FLUID, so that it has no fluid states.
2. Split up minecraft:flowing_FLUID into minecraft:flowing_FLUID with the fluid states level=1-7, and minecraft:falling_FLUID without fluid states.

This will result in the following:

• minecraft:FLUID, no states, used for source blocks.
• minecraft:falling_FLUID, no states, used for fluids flowing downward.
• minecraft:flowing_FLUID[level=1-7], used for fluids going sideways from minecraft:FLUID or minecraft:falling_FLUID.