When a champ with an active 50% ATK buff gets a smaller 25% ATK buff, the latter replaces the former. This doesn't only make the 25% buff worse but makes it actively bad.

This is a recent(?) bug as it didn't happen when I used Valerie and Spirithost in the same team some time ago but it does happen with these champs now.

Tests with Jareg (60% DEF buff) and Bogwalker (30% DEF buff) show it doesn't happen with DEF buffs.