I have been working in a research position in a DOE lab in the United States for the past year or so, and my mentor wrote me letters of recommendation for my graduate school application. I have been, to put it bluntly, worse at my job recently than I had been when he wrote the letter, and I would quite like to leave soon as:
It has always been a poor fit for my interests, and I am rapidly losing the motivation to continue;
I have not been challenged/learned anything new for months, and I do not have the discretion to change this;
I am not vital to any of the ongoing projects in my group;
I have saved enough to support myself through matriculation.
I am very worried that, given my recent job performance, if I leave soon, my mentor might try to contact graduate programs to which I submitted applications 1+ months ago to give an update on his impression of my character/abilities/etc. Is this a real possibility? If so, how would graduate schools react, receiving this so long after submission, or if they've already accepted me?