Whether one should send just the 1040-X or both the 1040-X and the updated 1040 which reflects the numbers on said 1040-X is often subject to (spirited) debate on this forum.
The instructions for the the 1040-X specify that all documentation that support the amended changes need to be attached to the 1040-X. Does that mean the updated 1040? Probably not, but I am firmly in the camp of "send the updated 1040 with the 1040-X".
I include the updated 1040, with all schedules and statements, behind the 1040-X and write *Amended Tax Return* in red sharpie that the top of the Form 1040.
I have not have any problems yet, and have filed plenty of amended tax returns (for new clients thank goodness).
I wonder what will be transmitted when 2019 tax year 1040-X is efileable soon? Just the two page 1040-X, or the 1040-X and 1040? Food for thought...