That happens with some champs (I remember being annoyed years ago when it happened to my 4* Wolverine I believe). I can't say for sure it is intended with Star-Lord and don't know the full list of other champs it happens to but what I was told by mods in the past was that PI/Attack/Health can go down on rank up but when max level is achieved (or earlier) you will pass where they were on the previous rank.
The explanation made no sense to me but Mike claimed at the time that they had to decrease where you started from at the new rank so that the increases when getting to max level for the new rank would end where they wanted them (or something to that effect). I never understood why if they felt keeping you where you were at the previous rank to start the next one would lead to too high stats at the max level of the next rank they couldn't just decrease the amount of stat gains you got on each new level of the higher rank so you still finished where they wanted you.
It seems really stupid to ever make increasing a champ lead to them being temporarily weaker (especially since not everyone can put a bunch of levels in a newly ranked champ right away and especially now with escalating iso/gold costs per level in higher rarity champs) but that was the official explanation I was given and never was given any replies to address the reason they would make it work that way instead of what I had mentioned.
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The explanation made no sense to me but Mike claimed at the time that they had to decrease where you started from at the new rank so that the increases when getting to max level for the new rank would end where they wanted them (or something to that effect). I never understood why if they felt keeping you where you were at the previous rank to start the next one would lead to too high stats at the max level of the next rank they couldn't just decrease the amount of stat gains you got on each new level of the higher rank so you still finished where they wanted you.
It seems really stupid to ever make increasing a champ lead to them being temporarily weaker (especially since not everyone can put a bunch of levels in a newly ranked champ right away and especially now with escalating iso/gold costs per level in higher rarity champs) but that was the official explanation I was given and never was given any replies to address the reason they would make it work that way instead of what I had mentioned.