There is something wrong on UEFI_MULTI v4.5.
I recicled a 3 GB VHD with two partitions deleting both partitions and making a single NTFS partition with Win10 tools, then rebooted from Win10XPE_x64 to deploy the WIM file and decided to do it with UEFI_MULTI v4.5, to test this new version, but just after it mounted the VHD it stop working and showed a message, after acepting the message UEFI_MULTI v4.5 closed immediately, please see attached pictures.
On first picture I selected to keep the VHD mounted, and didn't make any other change, on second intent after reformating the NTFS partition with Win10 and unmount the VHD, I didn't select to keep the VHD mounted but selected 1 partition just to check if this time it was able to finish the task, but the result was the same.
The report is about VHD_WIMBOOT-45 and not UEFI_MULTI v4.5
I tried to reproduce, but in my case I don't get the message.
There was no Mounting VHD file related change in version 45 as compared to previous versions.
The message is normal to occur in case your VHD file was mounted in advance of using VHD_WIMBOOT.
In that case simply unmounting the VHD will solve the issue.
The 1 Partition setting is used in case of creating New VHD, but has no influence in case of selecting existing VHD file.
The Keep Mounted setting is used when VHD_WIMBOOT finishes to keep the processed VHD mounted, but that setting is not involved when you get the message.
In any case check what drives occur before using VHD_WIMBOOT and Unmount Drives related to VHD files.
May be you try again with other New VHD MBR 1 Partition Fixed VHD made with VHD_WIMBOOT.
Can you give VHD_WIMBOOT-45\makebt\wim_info\vhdlist.txt in case of trouble when you get the message
I will keep trying to get the message in case of Unmounted VHD, but until now it did not occur in my case.