ImpureMethodCall occurred for filling new "Ds\Set" with elements in method for class marked as immutable

[andrew-demb]

https://psalm.dev/r/50506eccbe

<?php

/** @psalm-immutable */
class Foo
{   
    public function getReferencesTo(): \Ds\Set
    {
        $references = new \Ds\Set();

        foreach ([1, 2, 3] as $id) {
            $references->add($id);
        }

        return $references;
    }
}

Psalm output (using commit [37cc4fd](https://github.com/vimeo/psalm/commit/37cc4fd)): 

ERROR: [InvalidArgument](https://psalm.dev/004) - 11:30 - Argument 1 of Ds\Set::add expects never, but 1|2|3 provided

ERROR: [ImpureMethodCall](https://psalm.dev/203) - 11:26 - Cannot call a possibly-mutating method Ds\Set::add from a mutation-free context

ImpureMethodCall is not expected for me, since I instantiate $references with new \Ds\Set() just at the start of the method.
Set::add isn't pure, but mutates a new object initialized just now.

Is it expected behavior in psalm for immutable method?

PS: I can't find the reason for the error InvalidArgument in the playground, but locally I haven't seen it.