After unasign a pc from a FDE policy the policy is removed from ZENworks agent, but not from the FDE agent.
The disk left un encrypted state. Only after manually remove the policy from FDE agent the disk is decrypted.
(FDE agent - Commands - remove policy)
Agent Version is 220.127.116.11, API 12.3.907.11
Windows 10 1903 Build 18362.30