The Diplomat's effect will only take action if:
1. The Diplomat is at an Outpost and NOT in a sub
2. The enemy outpost where the specialist is must be within the range of the Diplomat, NOT the prisoner sub, which can move out of the range of the Diplomat if the enemy's nearest outpost is that far away.
There is also a possibility that the first game was before an update which happened last year- before that update, the Diplomat would immediately release a specialist from a prisoner sub, but now the prisoner sub must arrive at an enemy outpost to be released.
before and after screenshots would help clear it up.
Reporting from the Bridge