Hey together! Possible a „solution“ for all, who are able to boot into EFI but not into Windows anymore after a Surface-Firmware Update:
I have quite a similiar issue after i applied the newest „ Surface - Firmware - 235.3261.768.0“ (UEFI) and the „ Surface – Firmware – 241.304.139.0“ (Aggregator) on my SP6.
After reboot it hopped into UEFI. I tried everything: no USB-Boot and so on. Tired original Surface BMR Recovery with thausands of different USB-Sticks, disabled secure Boot... Nothing. Just for fun i had a Ubuntu 20.04 Install-Stick.. well, and this one
bootet up. For testing i installed it with all neccessary Kernels and Drivers. All was working fine. No faulty SSD or somethin. But Windows still dont wanted to boot. Used several Unix-Tools to get the damn Windows to boot up on a SURFACE.
After hours and hours i finally found out, that after the UEFI Update the UEFI-Part if my Surface will not accept Windows Bootloader-Files pre Windows 10 era.
So i downloaded the latest Windows 8.1 Build, extracted the bootx64.efi and bootmgfw.efi (is the same File but different naming...) and overwrite them on the EFI Partition/USB Stick /EFI/Boot and /EFI/Microsoft/Boot.
And voila: My Windows Surface-Starts up. I had stock 1903, after 2004 the Error reappears after the first reboot (restored this efi Files from !!2014!! again) Same thing from 2004 to 2009 (20H2). I am hoping MS releases a fix for that! ...so that my Surface
is able to boot Windows 10 (and future Featureupdates). I dint know how long my old-efi File trick will work on future Builds.
conclusion: it seems that MS breaks on some devices with the Surface-Firmware Updates some Bootstack elements in their UEFI.
normally without this efi-Files, my surface will be a non-Windows running, Linux-booting brick!! Weirdoo..