Apparently there's a bug introduced by some OS upgrade. The grub menu is completely skipped under Fedora 32. No grub menu at all even though Windows 10 is detected there's the default 5-second time out for gurb menu display.
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Unlike other distros like Ubuntu, major Fedora upgrades overwrite existing grub file. Next time you boot your computer, all other operating systems are gone and you boot straight into Fedora. The solution is one line:
grub2-mkconfig -o /boot/grub2/grub.cfg

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