I've tested it on my end and indeed it seems like leaving an empty space will result in router always pinging to 188.8.131.52 by default from whichever interface is enabled in the failover configuration. I'll make sure to forward this information about the issue to our RnD department.
As a workaround you can enter a value of "0" or "0.0.0.0" (either will work) in the address field. This way the router will keep on pinging itself which will not consume any internet traffic and will make sure that interface is considered as "Always up" as long as it is enabled.
Alternatively, you could increase the interval between pings on mobile interface but this will still consume some data.