Hi Sythen -
I have attempted to reproduce this issue internally and am able to import All for 100 Static Meshes with only hitting the Import All button once. I tried also adding in Morph Targets and Skeletal Meshes to make more than 100 files and only when the Import hit a different file type did I get another pop up. Can you provide anymore information as to the type of project (have you tested it in a Blank Project) and what type of assets you are importing.
Thank You
Eric Ketchum