Summary:

A downwards flow of lava is cut off by extending pistons. When the pistons retract, the lava can sometimes instantly flow two blocks down. 

Steps to Reproduce:

1. Construct a vertical tube of flowing lava 3 block tall

2. Place 2 pistons in bottom 2 blocks of one of the walls of the tube such that when extended, the pistons will cut off the flow of lava but leave the source block at the top of the tube.

3. Waterlog the bottom piston, and obstruct the sides and rear so that the water is directed to flow out of the head of the piston.

3. Power the pistons, causing them to extend and sever the flow of lava

4. Unpower the pistons, causing them to retract. 

Expected Result:

The pistons will retract, the waterlogged piston will flow into the bottom block of the tube, and the lava will slowly flow back down the tube at a normal rate, meeting the flowing water and creating a stone block. 

Observed Result:

Sometimes, the lava will instantly flow down the tube, filling the tube before the water can flow out of the waterlogged piston.

Notes:

I was unable to recreate this bug without the presence of the waterlogged piston, it appears to me that some sort of flowing water at the bottom of the tube is required for this bug to occur. This bug is particularly relevant in the context of a stone generator, as when the lava flows instantly (which occurs a good portion of the time), it cuts off the flow of water and does not create stone, causing the generator to be inconsistent. 

See the attached videos of how the system behaves as expected vs how it behaves when the bug occurs.