Replace that with the following and it will work perfectly:

public function setPnAttribute($value)
{
    $this->attributes['pn'] = normalizer_normalize($value, Normalizer::NFKC);
}

About this part of the code: strtoupper(trim(substr($value,0,50))). I recommend you modify the string only after you normalize it so you have a correct and desired output. Otherwise use mbstring instead of the default php functions.

Edit: What i mean by the last part is that the following two examples will always produce correct code:

public function setPnAttribute($value)
{
    $this->attributes['pn'] = strtoupper(trim(substr(normalizer_normalize($value, Normalizer::NFKC))));
}

public function setPnAttribute($value)
{
    $this->attributes['pn'] = normalizer_normalize(mb_strtoupper(trim(mb_substr($value))), Normalizer::NFKC);
}