After looking waaaay more carefully at the block - I do see what the original poster means - especially on the bottom miter.
There also seems to be some puckering of the fabric against the outside strips - so - perhaps the inside block is "a bit too big" for the outside strips and needs to either be cut down a bit - or the seams need to a very tiny wider to "shrink" the block size. Or did the puckering occur when it was pressed?
Is that last seam a diagonal one through the center of the block?
As far as those last miter joins - for "marking" or pinning - I stick a pin vertically into the "exact" spot where the vertices meet - and then pin on both sides beside the vertical pin - and remove the vertical pin.
The reason I do not leave the pin in the "exact" spot is that when I "lay the pin on its side" - there is a bit of distortion and then the seam gets more "off" then when I insert the "holding pins" on each side of the "exact" spot.
Hand basting of the join is another possibility. It's sometimes worth the effort!