Thank you for your summary of the study. While I’m not sure why exactly the triangle isn’t appearing, you can manually set the pointer to be in the middle of the slider by adding a custom code component and under Begin Routine adding
slider.markerPos = 0.5 (I think this should work if you have the granularity set to 0).
That said, because your task seems to be a two-alternative forced choice task, it might be better to set the slider granularity to 1, making the scale discrete rather than continuous.
Hope this helps.