Don’t know why I spent the last hour poking around GRBL firmware code and circuit diagrams even though I have not used GRBL or am familiar with this board.
I have just come across folks who work the with the Z limit switches. But the probe command is something else. So would that be on the AUX2 A5 pin? I’m not real familiar with all these firmware instructions and pin outs. But sometimes I just start wondering out loud and someone who knows something will step in and correct me.
This reprap topic might give you some idea of what the issues are, as in, not well documented board.
https://reprap.org/forum/read.php?13,753796
This post might help as it contains a schematic of the whole board. Is this where Analog 15/ Digital 69 is? Its Extruder 1 (ord the second installed extruder) temperature probe connection.
Here is a clearer picture of the board.
https://klipper.info/mcu-resources-1/mks-gen-v1.4
Anyway, I don’t know if I helped at all, but at least in convinced me that there is a lot more to learn.
So if you can get a different pin set up that would make sense.