False positive PhanCoalescingNeverNull in multi-dimensional array access in 2.4.7-dev

[TysonAndre]

<?php
class Example {
    const X = [
        'foo' => [
            'a' => 'b',
        ],
    ];

    public function main(string $key) : ?string {
        // False positive PhanCoalescingNeverNull Using non-null self::X[$key]['a'] of type 'b' as the left hand side of a null coalescing (??) operation. The right hand side may be unnecessary.
        return self::X[$key]['a'] ?? null;
    }
}

Expected: Phan should infer the real type of self::X[$key] is nullable, and not warn.