This page contains the list of deprecations and important or breaking changes for Vault 1.2.1 compared to 1.2.0. Please read it carefully.
AppRole Upgrade Issue
Vault 1.2.1 contains a known issue where an existing AppRole role may not be read or updated under a specific scenario. The role in question needs to have been created before Vault 1.0.0, and then updated between 1.0.0 and 1.1.5. If using AppRole, do not upgrade to until his issue is fixed in 1.2.2.