The same thing happened to me as well; the gimbal will drift while it's indoors on the ground. My guess is that the gimbal is trying to fight itself. But when it's outside, it doesn't drift at all. Don't worry, nothing is wrong with your Spark
About 15% of Spark's propeller are actually blocked by spark itself. You see, these winglet design spreads out the downwash of those props. The current spark props design pushes wind straight down. So producing a wider downwash would instead be blocked by the top of Spark's body. Furthermore...
I think I've figured it out: tried it in another location and as soon as it says "The homepoint has been updated" it starts to ascend higher than the 5 metres limit.