You likely need to tell the uefi software to boot Grub. I can’t remember the command off the top of my head sorry but you basically need to tell it what to boot by default. Then you can let Grub handle the choice of Linux or windows. I just set up a laptop for my sister that behaved that way. No matter what I selected as default in the uefi setup it kept resetting back.
Just looked it up, efibootmgr is the command I think. https://www.linuxbabe.com/command-line/how-to-use-linux-efibootmgr-examples
Sometimes they need to be upset before they realise what they did wrong.