For joystick buttons assigned with SimConnect_MapInputEventToClientEvent the UpEventID is never transmitted.
For example for this call:
SimConnect_MapInputEventToClientEvent(hSimConnect, INPUT_0, "joystick:1:button:8", EVENT_REV_1, 0, EVENT_REV_1_RELEASE,0, true);
the EVENT_REV_1 will be transmitted on button press, but the EVENT_REV_1_RELEASE will never be sent.
For comparison, mapping keyboard keys works fine and both events are transmitted in this example
SimConnect_MapInputEventToClientEvent(hSimConnect, INPUT_0, "a", EVENT_REV_1, 0, EVENT_REV_1_RELEASE,0, true);
This was also bugged in FSX but in a different way. In FSX the "down" event was transmitted for a second time on button release. In MSFS, there's nothing, so we can't even implement a workaround.