My last name has an "ö" in it. That doesn't work in 99% of so-called global websites, so I have to transcribe it as "oe" instead, which is just wrong.
My husband has three first names and one last name. However, his actual name is his second first name. If his full name is "A B C D", if you ask him what his name is, he'd say "B D". If you know him you can call him "B", if you don't, you can call him "Mr D" or "Mr B D". Most American companies call him "Mr A" or "Mr A D", which is just wrong.
I have a friend with two first names and two last names, but his second first name is his name, and he prefers his first last name. If his full name is "A B C D", he'd say his name is "B C" if you ask him. Calling him "Mr A" is just wrong, and calling him "Mr A D" is super duper wrong.
(In government systems in our home country, this is of course handled. You can "mark" which first name(s) are preferred, so mail to my husband from either government or private companies will always be correctly addressed to "B D" and nothing else.)