I tried "What do you call your grandfather" knowing that Cómo se llama usually means "what is the name....", but asking a person what they call a grand parent seems like a common question. Our grand daughter calls me 'granpa and my wife lita. SO how would a person ask the name they use for their grandparent?
I had the same thought, but after considering it, realized that the problem comes from the mix of conjugations "se llama" and "tu". I think if we wanted to ask "What do you call your grandfather" we would need to change "se llama" to "te llamas" If we leave it as "cómo se llama" I think it more directly translates to "What is subject called?" because the conjugation is not second person familiar (although it would technically work for second person formal, but I think that is unusual and with a lack of contradictory context we are expected to stick with the traditional translation). I'm not sure if I explained myself very well, but those are my thoughts.
(And btw, I tried it, and "What is your grandfather called?" was accepted)
"Se" is a reflexive pronoun that can mean yourself (formal), himself, herself, it, you all, or themselves. (It is the reflexive pronoun that corresponds to the subject pronouns usted, él, ella, it (no Spanish translation), ustedes, ellos, or ellas.)
'Reflexive pronoun' + 'llamar' is a pronominal verb meaning 'to be named' or 'to be called.'
Me llamo Juan. = My name is Juan.
¿Cómo te llamas? = What is your name? (literally, How are you called?)
¿Cómo se llama? = What is your(formal)/his/her/it name?
Nos llamamos Salma y Maya. = Our names are Salma and Maya.
¿Cómo se llaman? = What are your/their names?
Duo introduced these phrases in the greetings lesson without diving into the grammar rules to get users familiar with asking someone their name.
Without the reflexive pronoun, llamar means to call.
¿[Tú] llamas tu madre todos los domingos? = Do you call your mother every Sunday?
The indirect object pronouns le and les change to se when followed by the direct object pronouns lo, la, los and las. https://www.spanishdict.com/guide/direct-and-indirect-object-pronouns-in-spanish
DuoLingo is a free service that is offered to help people around the world learn new languages. So if you are unhappy with the program, it doesn't make sense to be angry at them about it considering you are using it for free. Also, the people working at Duo have millions of sentences to proofread and troubleshoot and edit, so you can hardly be surprised when you run into occasional glitches that have not been addressed yet.
What's your grandmother's name? vs What is your grandfather's name? They are both correct however the latter is marked wrong. There isn't a difference. They're both correct and a contraction such as "what's" should never take precedence over proper English such as"what is".