Case 1 makes sense.
In case 2 I suppose the enemy on the bottom is not able to spot you due to lack of VR, while his LOS is still able to get to your tank, hence why you're still kept rendered for a vew secs. Makes sense to me as well.
Case 3 seems more like a bug to me because this does not make sense at all. I mean this implies that the enemy spotting ray is able to penetrate the oil tank, which should not be possible.
As expected with the recent spotting changes you get spotted faster than before, if you keep playing as usual. In my opinion in case 2 you should disappear right away when popping smoke, as long as no enemy is able to spot you. This would make more sense to me. If the bottom enemy is able to spot you, you should remain spotted.
For me personally the spot indicator after popping smoke feels very counterintuitive.