Created by: MartijnCuppens
Background images are removed from active buttons. This was initially done to remove the gradients in Bootstrap 3. In v4 this behaviour was moved to scss/mixins/_buttons.scss and only applied when gradients are enabled.
This code was never removed in scss/_buttons.scss:
https://github.com/twbs/bootstrap/blob/29f178d89129732d35ae812667709c64cdb28a6f/scss/_buttons.scss#L43