What’s going on in the FA-affected heart?

A particular thing about these hearts is that, over time, they maintain good systolic function [contraction of the heart muscle]. That means their ability to pump blood out seems to be OK — but because their hearts are thick, their ability to relax and accept blood to fill them is not OK.
They develop what’s called diastolic dysfunction [the resting phase of the cardiac cycle, between contractions of the heart muscle]. As a result, they lose the ability to adapt to the demands of exercise or increased blood volume.
The FA-affected hypertrophic heart adapts to increased demands by beating faster. That’s the only way it can adapt since it can’t increase its capacity to accept more blood with each cardiac cycle.
If the heart keeps having to beat faster to meet various demands, that can lead to a second problem, which is an irregular heart rhythm, or arrhythmia.