well that solves the mystery

The grubs in your beetroot are beet leaf miner. Usually just leave them alone but squidge if you want to. They will soon stop feeding and drop into the soil to pupate. There will be another generation in late August which then pupates to overwinter in the soil. When the flies emerge in the Spring they look for the nearest beet. Hence rotate your beet.

More of a problem in perpetual spinach and chard where their tunnels in the leave can spoil the crop.