On my bus what makes me think the sealant on the bottom of the windows was from the factory is, the sealant was all uniform down the entire bus. It seemed to be applied at the same time down every window base. There is a lot of it in every corner deep inside the corner that give me the impression they they put the sealant down then installed the window.
I have only tore into my bus so my experience is just that one bus.
In this video they show the window frame at 2:37
you can see the fold(and gap) on the bottom corner of the frame. That's how my window frames are, at the inside bottom corner there is a gap. That's where my leaks were coming from. Water would pool on the window ledge then drain into the corner.
Thinking about the window frame also made me wounder if the gaps could allow for flex. Which makes me wonder if wielding them might possible remove flex relief points.