Until now, both beets and weeds had hardened off, but the (light) rain of today and tomorrow ensures that the wax layer of the beets and weeds becomes much thinner and softer. This makes the weeds more sensitive to spraying, but so do the beets!
When it has actually rained and we will have to deal with a higher RH in the coming days, coupled with high radiation, then weed control is recommended. did not to be performed in the early morning. Try to postpone the spraying. If that is not an option, spray only in the evening and adjust the dosages downwards.