I think many of these "observations" are related to one thing.
Please, DEV of Prestige I have an idea what it might be.
I think the problem is the missing connection, or wrong connection of the Yrvantis Hollow Dungeon, maybe not just this.
In the cases from kleos, mine and Mela, it all leads to "15 missing achievement- counts in the card upgrade achievement".
The problem so far only exists in Candara continent counts.
To test this, you might do the following:
Step 0:
Please wait before you upgrade all the individual zones with diamonds/purchases.
Explicitly DO NOT upgrade Yrvantis zone with diamonds.
Step 1:
Upgrade 14 zones from Candara Continewnt to 1-star, i think you can use diaonmd purchase, just do this step.
This will lead to 70/75 completing in the continent achievement.
Step 2:
You can get a card from Yrvantis DUNEGON, not the zone, get one from the dungeon. (Taffrock?) In german Südliches Stahlfels.
Upgrade this to 1-star.
You will ckeck the continent achievement, and see, it will NOT count.
However, in the zone achievement for Yrvantis, it will count.
Step 3:
Upgrade any card from any Candara zone to 2-star, only one...
Check continent count.
I think it might now jump from 70/75 to 71/75 (still in the 1-star upgrade), although you upgrade 2 card to 2-stars.
I can better explain via direct DM on Discord if you want.
You can also check the Balanzasar continent count at same time....
In the end, Yrvantis, fully upgraded will have 15 individual achievement counts, which will not count to Candara, but Balanzasasar.
This results in the end to the missing 15 5-star achievements.
You may not believe me, it's ok, so just try it, or contact me in discord, we can check together.
I can not retest, as my progression is already "done mostly".