Hi, I have a very similar problem. I have two RAID 1 volumes Volume_1 and Volume_2. Volume_1 is 2x 4TB RED NAS Drives, while Volume_2 is 2x 3TB RED NAS Drives. The Share just dissapears if you Click on anoter Menu Item and then back to Shares.
I replaced the 3TB drives with 4TB drives and all was fine again. So doing many things, I ended up with re formatting the 3TB Drives many times (Through the Utilities Menu) and also when creating the RAID 1 Volume_2 again it did another Format.
Summary: If the two Volumes are each made up with 2x 4TB Drives all is OK But if you have 2x 4TB Drives and 2x 3TB Drives, you get the issue.
I have opened a Case with WD Support and as yet no info after passing them my results. Looks like this issue has been around for a while. I not sure what the case is if you use different sized volumes to the ones I did my testing on. Best of luck!
You can create shares on those volumes if you Shutdown and remove the Disks for Volume_1 and power backup. You will have the Volume created earlier as Volume_1 (even though it uses Bays 3 & 4) and you can creat Shares to your hearts content. Once done Shutdown again, reinsert the Drives in Bays 1 & 2 and power back up. You will now have the shares that you wanted. However, you still cannot create any shares on Volume_2 as in my case.