The mitres opened up because the water In the glue expanded the wood across the grain more than along the grain. This is the same reason that mitred corners are rare on well built boats. I think you would have been OK if you had overlapped and through cut them like you did in the last blog entry. I’ve done that with success a number of times. These are wide mitres though and might cause concern even then.
The early bird gets the worm but its the second mouse that gets the cheese.