We checked Group Policy Windows Update settings, all was configured correctly however computers still rebooted.
After troubleshooting further, we found that a deadline in WSUS was set to "Same day approval at 5:00AM". This meant as a deadline was set at 5:00AM in the morning, as soon a computers received the update upon boot, they already missed the deadline and immediately installed without prompting users to postpone the reboot.
We removed the setting for same day approval and this resolved the problem.
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