When you check x4 on the collection screen, there are many cards that do not display even if you have four of them.
It seems that cards that you had more than two versions of and a total of five or more of before the integration update will no longer be displayed.
example
Before update
Possessing 3 copies of illustration A and 2 copies of illustration B of the same card, for a total of 5 copies
↓
After update
You will have four copies of both Illustration A and Illustration B.

If you check x4 on the collection screen, this card will no longer be displayed.