Once you lay the tank on its side and expose the coils airhole(s) it probably will leak cause you would be breaking the vacuum. It could be a drawback of using stock coils and/or the vertical coils. Maybe try with a Qtip soaked in juice give all the orings a wipe before putting the unit back together or remove all the orings, clean the area under and reseat the rings. The glass tube might not be true so change if you can the glass or flip it around and reseat. There is a few things you can try to hopefully stop the leaking.
We use the subtank mini with both the stock cage and bellcaps from Fasttech probably since they first came out. I don't use the stock coils, only the RBA. My wife always sets hers on its side and it never has leaked, could be a RBA thing or the rayon wicking or how I wick it...I dunno. Its not like I got only a couple I use but well over a dozen with at least 4-6 loaded with juice sitting infront of me all the time and none of them leak from setting on their side or sitting for a length of time.
So many complain about kanger leaking but back in the day when I started with evods, T3s, protank 1,2 and mini and now the subtank mini none of them have leaked for me or have had problems, also I was rebuilding the coils in all of them could have made all the difference in my kanger vape experience.
No matter what atty you own IMO using the stock coils suck so learning to rebuild those stock coils or using the RBA will make all the difference.