Validation for composite field in Vuetify - vuejs2

Question

Let say I have a form with 10 input fields. field3, field4, field5 are related - validity of field4's and field5's user input subjects to field3's value.
Scenario:
User enters a value into field4. Its validator method (bound with rules prop) is auto-fired.
==> validation failed, field4 is placed under error state.
User edits input in field3. Watcher for field3 value is triggered, programmatically invokes the validator method of field4: this.$refs.field4.validate(q4.answer)
==> validation passed, I expect field4 state to change from error to success, but it actually remains in error state.
After series of experiment, I believe Vuetify components have two set of validation state, one for auto-fired validation, and another one for manual triggered.
From Vuetify's API docs, I found a form.resetValidation() method but it reset the former state for all fields within a form, thus produced UX doesn't make sense for above use case. I couldn't find anything in API docs for resetting validation state of just the targeted field.
What's the proper way to implement validation for above scenario, without producing poor/confusing UX?
----- updated with relevant code snippet
My generic proxy input field that wrapping Vuetify's input field looks like the following:
<template>
<v-text-field
...
:rules="[validate]"
></v-text-field>
</template>
<script>
export default {
...
props: {
... other props
additionalValidations: {
type: Function,
default: null,
},
}, // props
methods: {
validate(input) {
let validationError = null
... generic validations
// Perform additional validations only if basic validations passed.
if (!validationError && this.additionalValidations) {
const result = this.additionalValidations(input) // returns either true or err msg.
if (result !== true) {
validationError = result
}
}
return validationError || true
}, // validate()
}, // methods
}
</script>
Custom validation logic that compares multiple fields input is defined in parent component (such as page), then passed to above proxy input field via the additionalValidations prop.