I designed a Lava stone generator in my private creative Redstone world. This device works perfectly there.

I built the same machine on my Realm and find that it does not work consistently, I will usually get one row of stone, but the Lava flow rate appears to decrease to a pace which breaks the machine. I increased the clock pulse width to 16 ticks and still the lava would not flow one block of distance (down vertically) in time. Currently I work the device manually and wait till I see the lava flow before ejecting the stone created. I can visually see that the lava flow rate is inconsistent. sometime dropping in 1 or 2 ticks but mostly hanging in the air for long periods before flowing down.

You can test this by blocking lava flow using a piston then retract the piston and observe how long it takes the lava to fill the block space in front of the piston.

This issue seems to only affect a realm.