Try changing the letter to Y and rebooting and see what happens. It seems something is resetting it if diskpart doesn't persist.
Another thing you could look at is gdisk and make sure the partition type code is correct.
Download it and then from an elevated cmd prompt in the same directory it is in...
If it is 2700 you'll not see it in file explorer for example.
GPT fdisk (gdisk) version 1.0.0
Partition table scan:
BSD: not present
APM: not present
Found valid GPT with protective MBR; using GPT.
Command (? for help): p
Disk 0:: 236978176 sectors, 113.0 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 94EB0CDB-062F-4EF9-BDED-04F144D52C7C
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 236978142
Partitions will be aligned on 8-sector boundaries
Total free space is 1273309 sectors (621.7 MiB)
Number Start (sector) End (sector) Size Code Name
1 40 1024039 500.0 MiB 2700 Windows RE
2 1024040 1433639 200.0 MiB EF00 EFI
3 1435648 1468415 16.0 MiB 0C01 Microsoft reserved ...
4 1468416 49500159 22.9 GiB 0700 Windows10
5 49501912 186482799 65.3 GiB 0700 Shared
6 186482800 235708599 23.5 GiB AF00 El Capitan
Command (? for help): q
You can change the partition code using the t and then w options.
Alternatively as other have said you could delete it.