That is a sign of a lack of maintenance....
After only ~100hours use?
The only way I can see that the (spring loaded) nuts can drift apart, as seen in the image, is if they were not in the Z-Screw housing from the start?
However, as the saying goes: "There's always a silver...". While refitting the Z-Nuts, I noted that the two (LM6UU) bearing were completely shot! Two new bearings fitted and walla.... Solved a long standing issue: See "What is happening here?"