Your spreadsheet has circular references. By itself, that is not necessarily a problem. In this case, your spreadsheet has iterative calculations disabled. That makes it a problem.
To fix it, open the spread sheet, click on File, click on Options at the bottom of the left column, click on Formulas in the pop-up window, and click to put a check mark in Enable iterative calculation at the top right. I have mine set to 100 iterations and .001 change.
With these options, changing Q2985 to =MIN(FILTER(Q13:Q2981,Q13:Q2981>0)) works as expected.