A few points from my understanding of the system.
The relay is driven by 12V. The system always drives the relay with 12V regardless of being operated from 24V. There is a reason for this which I won't elaborate.
The 51385 pump controller drives the pump with 12v, regardless of the 24V power supply. You may measure 24V with no load (pump) connected, but with the correct pump connected (and running) you should measure 12V.
If you measure 24V at the pump terminals while the correctly specified pump is operating then your 51395 has failed.
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