[Upgrade Issue] [solved]System boots into BIOS instead of Windows setup after upgrading from 21H1(RTM) to 25H2

[leohazy]

Background

Source OS:

• Windows 11 Pro
• Version: 21H1 (RTM)
• Build: 22000.1
• Architecture: x64

Target OS: Windows 11 25H2

Tool used: flyoobe V2.2

ISO: Win11_25H2_Chinese_Simplified_x64.iso

Steps & Behavior

Successfully patched the original ISO using flyoobe.

Ran the Windows upgrade installer from within Windows 11 21H1 — the upgrade process started normally.
After the required reboot, the system did not enter the Windows setup/update screen, but instead booted directly into the BIOS setup.
Manually selecting the Windows disk via Boot Menu / Fast Boot still resulted in booting back into BIOS repeatedly.

Workaround

Changed the boot mode in BIOS from UEFI/Legacy (Mixed) to UEFI Only.
After rebooting, the system successfully entered Windows.
Windows automatically resumed the 25H2 upgrade process and completed successfully.

Result

Upgrade to Windows 11 25H2 was ultimately successful.
However, the reboot phase failed to automatically boot into Windows Boot Manager until the boot mode was forced to UEFI only.

问题背景

原系统版本：

• Windows 11 Pro
• Version: 21H1 (RTM)
• Build: 22000.1
• Architecture: x64
  目标版本：Windows 11 25H2
  使用工具：flyoobe V2.2
  ISO：Win11_25H2_Chinese_Simplified_x64.iso

复现过程

• 使用 flyoobe 成功 patch 原版 ISO，生成 Win11_25H2_Chinese_Simplified_x64.iso。
• 在 Windows 11 21H1 系统内运行升级程序，升级流程正常启动并完成第一阶段。
• 系统提示重启后，并没有进入 Windows 更新/安装界面，而是直接进入 BIOS 设置界面。
• 在 BIOS 中使用 快速启动（Boot Menu） 手动选择 Windows 所在磁盘启动，仍然会反复进入 BIOS。

临时解决方法

在 BIOS 中将启动模式从 UEFI / Legacy（混合） 修改为 仅 UEFI（UEFI Only）。
保存并重启后，系统成功进入 Windows。
Windows 自动继续 25H2 的升级流程，并最终顺利完成系统升级。

结果

系统最终成功升级到 Windows 11 25H2
但在重启阶段出现 无法自动从 Windows Boot Manager 启动 的情况，疑似与启动模式或引导项有关。