I believe Zapier takes care of turning your value into a JSON string. Can you try removing the additional quotes around the outside (and the backslashes)?
Sorry to hear that you’re dealing with this issue.
It looks like the date you’re inputting is in the date format of “March 31, 2021”. Instead, it should be “2021-03-31,” or YYYY-MM-DD as specified in this section of our API documentation: https://monday.com/developers/v2#column-values-section-date.