You are correct about coral reefs existing in warmer places, however the issue is more nuanced than that.
Corals have a fairly tight thermal range for which they have evolved to thrive in, it is when the water temperature exceeds these ranges that they become stressed and expel their symbiontic algae, for which bleaching is typically the result.
In the past decade, this has also affected corals along the equator where the water ois typically much warmer in the Maldives as well as Raja Ampat in West Papua to name some.
This can lead to less offspring being produced by colonies as well as an increased susceptibility to disease.
