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I have the START icon time filter set to "off"
and "close all positions after hours" set to "true"
and the TIME icon set to stop trading at 1600 hours.

So why has my trader not closed its currency pair?

The options in the START icon apply to the expert advisor. If in the START icon says OFF, the time filter options in the START icon won't work so no position will be closed. The START icon should be used to give a general trading filter for the ea, like trade from x hour to y hour and if you are out of that range close the open positions. Since it lacks of the day option we created the TIME icon.
The TIME icon should be used to add time filters joining it with TA icons (using an AND) so they work at the condition level. So you get things like open position if the TA condition is true and it's Monday between 9 and 11.
You can use both filters at the same time. The expert advisor will use the condition in the START icon first and then will check the TIME icons.