Unbound template type is not narrowed correctly about psalm HOT 2 OPEN

<?php

class BigIntType
{
    /**
     * @template T
     * @param T $value
     * @return (T is null ? null : int|string)
     */
    public function convertToPHPValue(mixed $value): int|string|null
    {
        if ($value === null || is_int($value)) {
            return $value;
        }

        assert(is_string($value));

        // remove decimals
        $dotPos = strpos($value, '.');
        if ($dotPos !== false) {
            $value = substr($value, 0, $dotPos);
        }

        return $value;
    }
}