The explanation was found at the following link.

https://develop365.gitlab.io/nuxtjs-2.8.X-doc/ja/api/configuration-watch/

e}xpg}woe,ar.nftt,eacrlhdal:etbfea[a:c'uksl{:ttatt{riuce',],

By providing watch in the nuxt.config.js file as shown above, the target directory became a watch target as well.