In 18.5 the imagemagick node in tops changed.
The convert mode doesn't have an “Output Filename” but now has an “Output File Path”
and it seems the {inputfilename} variable doens't do it's job any longer
instead it creates files with literally {inputfilename} as the filename and with multiple files overwrites each one with the next one
I just tried my setup and it still works in H18.
Is there some new workflow with this node I am not aware of or is it a bug?