+ operator over array<int, int> is giving me mixed value

[VincentLanglet]

I simplified my code to https://psalm.dev/r/1c3274bd0a
In my real case, I'm adding another array.

I don't understand why the value is considered as possibly mixed.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/1c3274bd0a

<?php

final class Token
{
    public const ONE = [
        16 => 16,
        21 => 21,
    ];

    public const TWO = [
        17 => 17,
        22 => 22,
    ];

    public const THREE = [
        18 => 18,
        23 => 23,
    ];

    public const RESULT = self::ONE + self::TWO + self::THREE;
    
    /**
     * @param int|int[] $bar
     * @return int|int[]
     */
    public function foo($bar) {
        return $bar;
    }
    
    public function test(): void {
        $this->foo(self::RESULT);
    }
}

Psalm output (using commit 3fb9e1c):

INFO: MixedArgumentTypeCoercion - 31:20 - Argument 1 of Token::foo expects array<array-key, int>|int, parent type array{16: 16, 17: 17, 18: 18|mixed, 21: 21, 22: 22, 23: 23|mixed} provided

[weirdan]

Funny thing that it works for (any) two arrays, but not for three.