Banner of Kinship miscount bug

Banner of Kinship has "When this support enters the board: All your creatures get +X/+X. X is the total shield of all supports you control."

This works great on the first cast of the card. For any subsequent casts of the card, it only adds the additional shields added to your board. In this example the first cast correctly added +21. The second cast on the next turn only added +7 since I had played a Mox (3 shields) and the Banner (4 shields). I've tested this a few times, and it plays consistently like this.
https://www.youtube.com/watch?v=-rkcfB6eKO8&t=5s