In your screenshot, you have span_frames_min_duration set to 5ms, and the span you were looking at was 1.3ms.
span_stack_trace_min_duration is replacing span_frames_min_duration but I'm not sure which version of the UI will have that change. The new setting will take precedence, but the old setting will still work if the new setting is not set.
If you want to see stack traces from all spans (including the short ones), you would set that setting to 0ms in Kibana.
Alternatively, you could configure it on the agent side, either via environment variables or in-code config.
Note that collecting stack traces has some performance overhead. We assume that most users won't want to inspect the stack trace on spans that are less than 5ms, because they are usually not the problematic spans. Thus, the default of 5ms.