Optional props in Vue 3 are not optional anymore on the resolved props value from defineProps:

• types(defineProps): remove optional properties from type core#6421

Additionally, Vue 3 casts optional boolean props to only type boolean, because of https://vuejs.org/guide/components/props.html#boolean-casting.

The only correct way to allow undefined for a boolean prop is to manually set it as a default with withDefaults. (see reproduction components/BooleanWithUndefined.vue)

This should also reflect in the <template>.

Link to reproduction