Hard to explain, if you press F11 while your mouse is locked it will only let you look as far as the mouse would normally go on the screen.
EDIT: You can prevent this by unlocking the mouse first (but it is still an annoying bug)
MC-63600
Pressing F11 while mouse is locked ingame won't let you see all the way around.
Resolved
Incomplete
0
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.