In both cases, there are no elements subject to compression and the instability and buckling in tension is related to the presence of the slider, the junction between the two rods, allowing only relative sliding between the connected pieces.
There are no elements subject to compression. This is in both cases. There are no elements subject to the instability. This is in both cases. Buckling in tension is related to the presence of the slider. This was in both cases. This slider is allowing only relative sliding between the connected pieces. The slider is the junction between the two rods.