So after installing openmamba you had the Windows choice but selecting it originated a "boot.mngr missing" message? If you (left) selected Windows as a boot choice during the installation it is not your fault. After openmamba (re)installation I would check that Windows partition is set as bootable. Other problems might depend on having more that one disk, but I don't know if this is the case. In any case a copy and paste of the installation log might help us to understand the problem. Also I will test again dual-boot installation to be sure that this is not a general regression of this release.

Thank you for responding. Being unfamiliar with this installer, it is possible I did not see the option to select Windows as a boot choice. Please can you indicate where this is presented during the install process?

The attached screenshot is the installer page for selecting non-linux (windows) partitions. All choices are preselected, so this can be the problem only if you unselected them by your choice. In this case anyway you shouldn't have got the Windows boot option and thus fall into the boot.mngr error, so I don't think this is the problem. If you would like to proceed with further testing, you may install openmamba again, and then we can try to detect and fix the problem from there.

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Many thanks for the detailed response. In fact I changed nothing in these settings so it appears something else is amiss. I will try to find some time for another install this coming weekend.