iOS app launch sometimes fail to stretch to full screen

hello,

i need some help.

mine app woks ok but sometimes on iOS11 (lets say once in every five launches) when launched its not fully stretched to fill entire display.

its properly alligned on top of screen but sometimes its not fully stretched so theres a black bar at the bottom of the screen.

this is OK

ID: 4040

and this is somewhat shrinked on sides and bottom.

ID: 4041

Help?