I’ve had a ton of issues with this, but I eventually found out that my blueprints weren’t updating after changing the C++ files they derived from, which led to empty details panels on some components and various Static/Movable mismatch errors.
The only way I’ve found to update them is to change the parent to something basic like Actor, then back to Tank/Turret, which resets it as if I just created a brand new Blueprint. so have to re-set the sprites/rotations etc.
http://image.prntscr.com/image/b7ab98b20bcb42708cd5a620db395345.png
Also, not sure if it was really needed, but I also followed a workaround posted in an Answerhub post for the ChildActor parenting issue which suggested to set the Child Actor in Construction Script instead of the Component details panel.
http://image.prntscr.com/image/f98afe7d18794d9d866f7c95c341f74a.png
After this, all was well in the world!
http://image.prntscr.com/image/6feeb772ecf24668bdfd2d5a8763c073.png