I have seen this before. There are two things that can cause this.
1) the windscreen setup is too tight. Esbit can into a thermal runaway condition similar to an alcohol stove, so good ventilation is important.
2) Small burn volumes can overheat the stove. By burn volume, I mean the volume of airspace between the ground to the bottom of the pot. The stove puts out ~325 watts. In a mug size system, this volume is about 24 cubic inches. With a beer can this drops to ~15 cubic inches. When you try to pump 325 watts out through half the volume, the interior "airspace" getts hotter. So, the smaller the diameter, the more ventilation you need. Alcohol stove are even more prone to thermal runaway and this is why it is harder to build a stove for small diameter mugs/pot. Best regards - Jon