I found a bug, (I think?) I was playing the web version on poki.com and I restarted, but after I had done that I was able to tap where the cooking upgrade was last time, and roll back an upgrade from the last run (this was before I had invented cooking in this run) and it gave me an extra 20 workforce. Now my total pop is shown as 31, but I have 18 free and have spent 33 (sorry this is so long-winded and confusing)