I am not sure about that.
Even though I don’t have any explanation why Windows Boot Manager had an EFI path pointing to grubx86.efi, I am pretty sure that your Windows was installed in Legacy/Bios mode. Reason: the absence of the mandatory ESP.
More, in Legacy/Bios mode, the Linux system as second OS will overwrite the MBR of the disk and cosequently overwriting WBM. This is the normal course of events. When the second OS is removed the WBM needs restoring.
This should be possible. I’m not sure what might have gone wrong.
I’m glad that you finally got your dualboot system up and running.
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