No one has ever really "caught" Celine. Even if it is kinda noticeable, she has never been caught on an awards show or on national TV (that I can remember). The reasons Beyonce, Britney and Mariah all got flack was because they flubbed in front of the world.
Artists like Beyonce and Celine, we know they can sing well naturally and use pre-recorded vocals only for casual assistance. Mariah got caught lipping when her natural voice was in the gutter, so that's why she got dogged on so much. And everyone knows Britney isn't the strongest vocalist, so that's why she gets the worst of it.