I also see the same behaviour where the second small partiton is of type EF ( EFI system partition ). I do note that enumerating all volumes using win32 api shows all volumes:
Drive(0,Samsung SSD 960 PRO 2TB), size=1907 (Gb)
Drive index = 0, partition index = 1,Volume size = 1907 (Gb), Locked=0,(DriveF,F:,NTFS)
Drive(1,Samsung SSD 960 PRO 2TB), size=1907 (Gb)
Drive index = 1, partition index = 1,Volume size = 1907 (Gb), Locked=0,(DriveE,E:,NTFS)
Drive(2,NVMe INTEL SSDPEDMW01), size=1117 (Gb)
Drive index = 2, partition index = 1,Volume size = 300 (Mb), Locked=0,(Recovery,,NTFS)
Drive index = 2, partition index = 4,Volume size = 194 (Gb), Locked=0,(Win2016,,NTFS)
Drive index = 2, partition index = 5,Volume size = 194 (Gb), Locked=0,(Win10,C:,NTFS)
Drive index = 2, partition index = 6,Volume size = 450 (Mb), Locked=0,(,,NTFS)
Drive index = 2, partition index = 7,Volume size = 727 (Gb), Locked=0,(DriveD,D:,NTFS)
Drive(3,HGST HUS724040ALE640), size=3726 (Gb)
Drive index = 3, partition index = 2,Volume size = 3725 (Gb), Locked=0,(DriveG,G:,NTFS)
Drive(4,Envoy Pro SCSI Disk Device), size=447 (Gb)
Drive index = 4, partition index = 1,Volume size = 48 (Gb), Locked=0,(USB1,H:,FAT32)
Drive index = 4, partition index = 2,Volume size = 398 (Gb), Locked=0,(USB2,I:,NTFS)
Drive(5,Patriot Memory USB Device), size=235 (Gb)
Drive index = 5, partition index = 1,Volume size = 235 (Gb), Locked=0,(Win8.1SE,J:,NTFS)
Drive index = 5, partition index = 2,Volume size = 0 (Mb), Locked=0,(UEFI_NTFS,K:,FAT)
But diskmgmt does not show small volume K:. Have not seem any instability in diskmgmt.