Sorry and regrettably, since the vault transfers are in beta, it is a bug. Also unfortunate is that Bungie cannot restore items lost for whatever reason.
I hope it wasn't important, but I send my condolences.
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Unfortunately, it was important. I also do not see anywhere in the version description on the App Store describing the 3.1.6 as being "BETA" with a "use at your own risk" policy. That sounds rather sketchy.