What you are describing is the first time I have ever seen this kind of issue and is completely contrary to how payment is set up:
How apps, content, and subscriptions from Apple are billed - https://support.apple.com/HT201359
"Some purchases aren’t charged to your Apple ID balance
If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method."
Family purchases and payments - Apple Support - https://support.apple.com/HT201079
"Which payment method is charged for family members' purchases?
One adult in the family—the family organizer—pays for most purchases that family members make. But if you're a family member and you have your own Apple ID balance, your purchases bill to your personal Apple ID balance. "
If you really, really, really are seeing your personal credit charged when another person in the family makes a purchase then report this to Apple:
Contact - Official Apple Support --> https://support.apple.com/contact
or:
Contact Apple for support and service --> "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232
