I did lots of testing in different projects . It seems that you right that the memory leak is inside the ImageMagick library . I used Ants Memory Profiler and it showed me that 99 percent of the memory my application is using is related to unmanaged code, and not dot net; the only library I have installed is the Magick.Net so something should be in ImageMagick.
Did you have a look into this ?
Did you have a look into this ?