Position of the head/nozzle is loaded into UI only after commands G28, G29, G30, G33. Just like /usr/local/Repetier-Server/firmware/marlin.xml says:
<command reportPosition="ask">G28</command>
<command reportPosition="ask">G29</command> <!-- not supported on Prusa i3 MK3/S -->
<command reportPosition="ask">G30</command> <!-- not supported on Prusa i3 MK3/S -->
<command reportPosition="force">G33</command> <!-- not supported on Prusa i3 MK3/S -->
<command reportPosition="yes" move="no">M114</command>
..
<command type="reportPosition">M114 ;@syncMotion</command>
So, entering G29 into console, MK3S will reply
Recv:17:38:24.045: Unknown G code: $117 G29
Recv:17:38:24.045: ok
Recv:17:38:24.049: X:62.00 Y:6.00 Z:0.15 E:0.00 Count X: 62.00 Y:6.55 Z:0.15 E:0.00
Recv:17:38:24.053: ok
and coordinates are loaded into UI.
But not when command M114 is entered into console.
Prusa i3 MK3/S (f/w 3.10.0) on Repetier-Server v1.0.4