It seems like $node ?: $this->nodeInstanceByValue() can also be of type Mbh\Tree\Traits\Node; however, parameter $node of Mbh\Tree\Builder::setNode() does only seem to accept Mbh\Tree\Interfaces\Node, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation