Both scenarios are the same, just different steps to get there but the steps do not matter much. It is simpler:
- add a clip to timeline
- trim its out point
- seek to the start of the clip
- press O
- the clip becomes full length
It should be either ignored or make the clip a single frame. A single frame is valid, but probably not what the user wanted resulting in a surprising glitch when viewing the export made worse because single frame clips are so hard to see on the timeline.
But part of your confusion comes from not seeming to be aware of current track and that it has a major role in this along with playhead rather than selected clip. The workaround to this problem is to change the current track to the one containing the clip you are targeting:
However, your examples illustrate probably a related bug: when moving a clip to another track it does not change the current track to the target track. The clip you dragged is still selected, but the current track did not update. Meanwhile, simply selecting a clip on another track does change current track. That is inconsistent.