So today I was playing Ryze and bought a Manamune which got to about 650/750 stacks, I then bought an Archangels and it seemed the stacks then applied to both Manamune and Archangels.

However once the stacks hit 750, Archangels was upgraded into Seraph's and Manamune's stacks were reset to 1, rather than Manamune being upgraded into Muramana.

Is this a bug or is this intentional? One would think if I got the stacks first on the Manamune then that should upgrade into Muramana first..