After extensive testing, I've concluded that the bug happens when there is about 350-400 milliseconds between clicking the LMB for the first and second attack. The sweet spot may vary with the speed of your system. Leaving this info here in case anyone else comes searching.
Not just "small" enemies. It happens against large slime as well.
... and Shieldwall Mitachurls.
After extensive testing, I've concluded that the bug happens when there is about 350-400 milliseconds between clicking the LMB for the first and second attack. The sweet spot may vary with the speed of your system. Leaving this info here in case anyone else comes searching.