V-trigger to S-trigger stops when devices share ground
The circuit you used to convert from Voltage trigger to Switch trigger connects 'ground' from the V-trigger source to 'ground' on the device being triggered. This works fine when both devices do have their grounds on these ports. 

But the Casio PT-50 probably has a keyboard matrix with neither side of the button grounded. Your hack connects one side of the One-Key-Play button to the LepLoop's ground. You may get away with this if you don't make any other connections between the PT-50 and the LepLoop (or any other equipment that shares a common ground with them), but as soon as you do a short circuit is created between the switch matrix and ground in the PT-50.  

To prevent a short circuit you need to break the electrical connection between input and output sides of the trigger circuit, while still passing the switch closure signal through. This can be done with an opto-isolator, which uses an LED and phototransitor to transfer the signal via a beam of light. Here is an example:-

     

The transistor in the 4N25 works the same as in your original circuit, except that it is turned on by infrared light falling on its Collector-Base junction rather than a direct connection. As in the other circuit the transistor only conducts in one direction, so you should connect the Collector and Emitter to the same points. 

The 1k&ohm; resistor sets the LED current, and the diode between pins 1 and 2 prevents damage to the LED from reverse voltage (it can be left out if you ensure that the V-trigger polarity is correct).