The activator rail check is done with the rail block a minecart was on before it was moved. This means if it moves from a normal rail to an activator rail the minecart is not activated in this tick.
- Build a setup as shown in Setup.png
- Place some items in the chest
- Put a hopper minecart on the powered / golden rail
- Right click the hopper minecart after it reached the end of the track
→ It picked up one item
Based on 1.11.2 decompiled using MCP 9.35 rc1
The method net.minecraft.entity.item.EntityMinecart.onUpdate() tests if the block the minecart was on before it moved is an activator rail. Instead it should probably test the block the minecart is on after it was moved (on rails or derailed).