Readonly behaves unexpectedly when using defined-or signatures

[plentyofcoffee]

Readonly v2.05

use v5.36; # enable strict, warnings, and signatures
use Readonly;

sub f($v //= '') {
    die 'empty!' if !length $v;
    $v;
}

Readonly my $s => 'test';
# chomp $s;
say f($s);

I would expect the code above to print "test". Instead, it dies with the empty! message. Uncommenting line 10, or otherwise using the variable as a string before passing it to the f function, produces the expected result.
The Readonly::Scalar function does not exhibit this same behavior.