I could have sworn this used to work. I don't know if something changed or I am forgetting something. We'll need to use the alternate method using the Update Item action instead of using an HTTP request. I like the HTTP request because it allows you to specify just the column(s) you want to update and you don't need to worry about the required fields. Do this instead:
The manual trigger will represent your trigger of when a forms response is submitted, and the get item action will be the get response details action.
The Apply to each adds each choice selected in the forms response to the array variable:
In the Compose action, insert the variable:
In the select action, set it up as show:
Use the expression item() from the expression builder to insert item in the right side of the map.
In the update item action, put your multiple-choice field into array mode by clicking on the icon to the right of the field name and insert the output of the Select action:
Hopefully someone will spot what I was doing wrong with the HTTP method, as I prefer it over using the Update item action especially if you have a list with a lot of required fields.