My child has Fire HD 8 (7th gen). When she is using it IN KIDS MODE, any purchases she has made (while in kids mode) are forgotten, including 'free' purchases. In-app-purchases while in kids mode is something you have to explicitly enable, so not a common use case, but seems like quite a few people are having the same issue:

https://uk.amazonforum.com/forums/devices/fire-tablets/480870-kindle-fire-has-now-lost-sons-purchased-minecraft