There is an issue where Camera rotation stops at game window border when using Begin play node with the mouse. This issue does not happen if you don't use the begin play node with the Set show mouse cursor node. It is unknown if this is intended behavior or not. This issue does not appear to be a regression.
4.16.3 - 3561208
4.17.2 - 3658906
4.18.2 - 3794801
Head over to the existingAnswerHub thread and let us know what's up.