If you fall into a one block tall lava source from specific heights (listed below), it will reduce fall damage, while falling from other heights will not.

Steps to reproduce:

1. Place down a single lava source.
2. Build a tower 13 blocks tall, including the block at the same Y level as the lava, such that there are 12 blocks above the surface of the lava.
3. (optional) Turn off fire damage and natural regeneration to better observe the results.

   /gamerule fireDamage false
   /gamerule naturalRegeneration false
   

4. Standing on top of the blocks, drop into the lava (without jumping). You should take 5 hearts of damage.
5. Add 1 more block to the tower, so it's 13 blocks above the lava. Again, fall into the lava. This only deals 2 hearts of damage.

Observed results:

Falling into lava from specific heights results in inconsistent fall damage. From my testing, fall damage will not be reduced from the following heights (including the block at the same Y level as the lava):
12, 13, 17, 20, 23, 25, 26, 28, 30, 32, 35, 37, 39, 41, 43
Other heights will reduce fall damage, up until 45 blocks when you will always die.

Expected results:

Going based off the resolution of MC-94166 (WAI) falling into lava would never reduce fall damage, not a mix of both.