Unique Sentinel removed after one

Yesterday I was running the first time a Unique Sentinel on Carcass Map (Specific for that map). I had run it with duplicated boss AND "Apex sentinel can Empower an additional enemy". Wanted to test if it is possible to duplicate the use like this. Because of the type of the boss (multi phase) it was not successful.

When I logged in today the Unique Sentinel was gone from its slot.
It still had charges.

The only reason it has more than 1 charge is to give you more chances of actually empowering the boss if you should somehow fail. You can only empower once though, like seaman said.