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Date & Time: 12/07/2017
Frequency: Always after exiting an instanced duty.
World name: Excalibur
Character name: Altena Trife
Class/Level: SAM 70
Party or solo: Both
Steps:
1. Summon minion
2. Enter instanced duty
3. Complete or leave instanced duty
4. Minion is no longer summoned.

■ PC Specs not applicable for this bug

* From my recollection, minions remained summoned after returning from an instanced duty pre-Stormblood expansion.

After the addition of minions being re-summoned upon log out and log in, one can only presume that this is a bug.