There is an issue when attempting to reorder blueprints where Blueprint Function Reorder displays error tooltip when trying to reorder functions. In previous engine versions, functions could not be reordered, but this new tooltip would indicate that in 4.18 functions could be reordered. This issue is not a regression.
4.17.2 - CL: 3658906 - Feature not implemented
4.18.1 - CL: 3754814 - Issue occurs
4.19.0 - CL: 3776583 - Issue occurs
Head over to the existingAnswerHub thread and let us know what's up.