How to reproduce:

1. Build the contraption in the attachments (both pistons are sticky).
2. Use bone meal on the grass block.
3. A grass/tall grass/flower appears and both pistons fire.

In vanilla the piston with the slime block on top doesn't fire.

I did some testing and this are the results:

1. The grass/tall grass/flower gets placed in the world with the "i" argument (i.e. the placement flags) equal to 2. This if-block captures it: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/nms-patches/net/minecraft/world/level/World.patch#144. Note that the flags argument is captured during the construction of the new CapturedBlockState.
2. Later on, I believe the captured grass/tall grass/flower gets actually placed from here: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/nms-patches/net/minecraft/world/item/ItemStack.patch#146.
3. It calls down to here: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/browse/src/main/java/org/bukkit/craftbukkit/block/CraftBlockState.java#217. Note that the flag field of CraftBlockState isn't used here. This eventually calls the method in point 1 again but with flags argument equal to 3 instead of 2.

I think the whole issue stems from the fact that the original placement flags are ignored when the grass/tall grass/flower is actually placed in the world.