New flat world, auto-recording disabled. Set randomtickspeed to 800, observe grass expanding, start recording.
In the replay, the first part of grass expanding is not captured and still shows as dirt while the grass expands beyond it.

2020_09_26_19_49_01.mcpr
2020-09-26 19-52-48.mp4.zip

Minecraft 1.12.2
Forge 14.23.5.2854
ReplayMod 2.4.4.jar
no other mods