Our kids' phones were setup to use my Store Apple ID (it’s an old practice — see a blog post about undoing this). The phones were set to require the Store Apple ID password prior to purchase. When our kids wanted to buy we’d enter the Store Apple ID password for them. Touch ID was not enabled for purchases.
After the 11.3 update two of the phones had Touch ID enabled for purchases. Normally you need to enter the Apple Store ID to enable this. One child (special needs) ran up a $10 bill which he paid for. (He gleefully shared his new power with us.). On one affected phone I turned it off again, when I turned it on the Store Apple ID was requested as expected. (Another phone I migrated to using its own Store ID, a third device wasn’t affected.)
On 1 of the 3 phones the Store Apple ID was changed to the user’s iCloud Apple ID. The problem was recognized when my daughter was unable to view movies not on her phone — it took me a few minutes to figure out what happened.
It’s interesting how much behaviors varied between the 3 devices.