before everything here some more information :

• i did enabled member intent on the discord developer portal
• my bot as all the needed permision on my test server
• and of course i have discord nitro myself to test it

@bot.slash_command(description="Generate your first card")
async def create_card(ctx: discord.ApplicationContext):
    user_has_nitro = ctx.user.premium_since is not None
    print(ctx.author.premium_since)

    modal = CardCreationModal(user_has_nitro=user_has_nitro)
    await ctx.send_modal(modal)

i tried using ctx.author and got the same result.
Ive got a weird interaction with another of my bot that as the exact same intent enabled and permision on the test server but worked somehow the only difference is the creation date between the two applications