You mean that if you define a fixed amount discount and you don't have this amount in products from the selected categories, the discount is not applied?
Cart rule:
Minimum amount -> 20€
Fixed discount -> 15€
Eligible categories -> Category A
Cart:
Products from Category A -> 10€
Products from Category B -> 20€
Total cart amount -> 30€
The problem is that discount is not applied because there are not at least 15€ in products from Category A