Learn Spanish (Latin America): Fue este aditivo lo que estuvo mal.

The first verb, fue (from ser), is used to identify what the culprit was—identifying things or pointing them out always uses ser. The second verb, estuvo (from estar), pairs with mal. In Spanish, being wrong or functioning poorly is treated as a state or condition, so it requires estar.

Why do we need lo que here instead of just que?

This sentence uses a structure designed to emphasize a specific point (known as a cleft sentence), literally translating to "It was this additive that which was wrong." In this emphatic pattern, lo que acts as a bridge meaning "the thing that." If you just used que, the grammar wouldn't hold together to give that emphasis.

Since aditivo is a masculine noun, shouldn't we use el que instead of the neuter lo que?

That's a great instinct, but lo que is preferred here because the "wrongness" refers to the abstract situation or the effect of the additive, rather than the physical object itself. It broadly means "the situation/factor that was wrong." Using el que here would sound a bit unnatural, almost like "the guy who was wrong."