Issue with`$targetIds` when saving a relation

https://github.com/craftcms/cms/blob/c75acb570df35b4593b89e06239f59f772e937b4/src/fields/BaseRelationField.php#L675

public function afterElementSave(ElementInterface $element, bool $isNew)
    {
        // Skip if nothing changed, or the element is just propagating and we're not localizing relations
        /** @var Element $element */
        if (
            $element->isFieldDirty($this->handle) &&
            (!$element->propagating || $this->localizeRelations)
        ) {
            /** @var ElementQuery $value */
            $value = $element->getFieldValue($this->handle);

            // $id will be set if we're saving new relations
            if ($value->id !== null) {
                $targetIds = $value->id ?: [];
            } else {
                $targetIds = $this->_all($value, $element)->ids();
            }

            /** @var int|int[]|false|null $targetIds */
            Craft::$app->getRelations()->saveRelations($this, $element, $targetIds);

            // Reset the field value if this is a new element
            if ($isNew) {
                $element->setFieldValue($this->handle, null);
            }
        }

        parent::afterElementSave($element, $isNew);
    }

$targetIds is at the moment required to be an array making impossible pass a model instance to the relationship

Start in src/fields/BaseRelationField.php at afterElementSave(), using the linked code to trace how $targetIds reaches saveRelations(). Confirm the expected relation input types and identify the relevant coverage before changing the save path. Done means a model instance can be passed for a relation without breaking array-based targets.