'editable-field' widget is not playing nice with `widget-value-set'

Question

When I evaluate the following form,

(defun magithub-issue-form ()
  (interactive)
  (with-current-buffer (generate-new-buffer "*magithub issue*")
    (switch-to-buffer-other-window (current-buffer))
    (widget-value-set (widget-create 'editable-field
                                     :tag "Title"
                                     :format "%t: %v\n\n"
                                     :size 76)
                      "Some default title")
    (use-local-map widget-keymap)
    (widget-setup)))

I get a buffer that looks like this:

Title: Some default title<editable field starts here>

widget-value-set works for other widget types (like checklist), so why does it misbehave for editable-field?

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A similar issue is happening for widget type text: