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As mentioned in the comments, it’s a reactivity issue. Only setting the whole value, using certain array methods, or using the .$set method will trigger the reactivity. You need to do something more like
const days = this.daysMatrix
days[indexProject][0].hours = 0
days[indexProject][1].hours = 0
//...
this.daysMatrix = days
or
this.daysMatrix[indexProject].forEach((value, index) => {
this.$set(this.daysMatrix[indexProject][index], 'hours', 0)
})
Source:stackexchange.com