We use [Vault 1.6.2+ent].
In both Java and .NET implementations, we had to add /data between the word secret and the rest of mount_path (using KV engine). EX: secret/data/applications/0077/non_production/development/.
This is not documented anywhere, it took me 4 days to figure this out after long search.
The question is, is this a bug? if not, is to stay in future versions?