Is it normal for the camera to capture a tilted horizon during flight? Of course this seems to be dependent on the wind, but I would've thought the gimbal would correct for this.

It seems to be "normal", even though it isn't supposed to do it.  I assume you mean just a small amount.  Many of us fight that.  When I notice it, I sometimes do a full rotation left and check it.  If it hasn't corrected, I do a full rotation to the right and check it again.  If that doesn't clear it, I make the correction in the gimbal manual adjustment.  I do with this was more reliable.

even though i also understand that is has to be fixed,  tilted horizon is normal. Of course  if it's too tilted , you have to do the gimbal calibration

It is not a good place to do IMU calibration

Old post but here's what I did.
First, I don't put it in the fridge just the fact someone mentions "condensation" on the camera tells you that's going with the electronics too.
water + electronics = dismay

1: Level surface,  turn it all on, controller and drone.
2: Turn on grid for camera (you will see grid lines in your camera view) these are too help orient you.
3: Do IMU calibration.
4: When it's done go to gimbal roll adjust (because IMU calibration alone does not fix it) adjust the gimbal roll so that the grid lines in your camera are horizontal and match something level in the view.
5: Do gimbal calibration.
6: Done, at least for me, the camera now stays level.

Your mileage may vary, but keep at it and you will find that one of the combinations of things here will fix it.

I have a tilted horizon also. But I can see mine try to correct itself. If I'm at say 200'and I look at the horizon it looks off but if I hover for a few seconds you can see it try's to correct itself but not fully. Anyone else have a slow response gimbal?