It's not the install, I just installed from a clean copy the latest Modloader and BTW (4.57), and it's been like this since 4.56.
The funny thing is, I can reproduce it on different hardware. I have a Gentoo Linux running Core 2 Duo with nVidia Geforce 210M graphics, where this occurs; I was able to reproduce it on a Core i3 laptop running Ubuntu (XFCE desktop) on both the Intel graphics and the nVidia Optimus GeForce 310M graphics sides.
Let me know if you need different angles on this. While it mostly occurs with block dispensers, I've had a similar effect hand-breaking flowers.