Flowerhorn aren't actually natural fish. They are a crossbreed of what people dont know what exactly yet. Similar to how a mule is. A cross between a horse and donkey. In this case a lot of issues can come up for them, As it is not necessarily perfected. It is almost a form of cloning. They were made by selective breeding, which is when 2 organisms are bred by a human or a way that they typically would not. Scientists are working on it are of now and trying to create new organisms by combining dna. That is selective breeding. Taking 2 dna and combining them to make an organism. There also needs to be a surrogate mother. For example: When a mom cannot have a baby she will find someone who can have the baby for her. But flowerhorns are an odd species as they ARE able to reproduce. But they were created by people using selective breeding so the unaturality of them is what can make them vulnerable to diseases.
Typically hybrids of closely related species are fertile, so flowerhorns being fertile isn't odd.
Species where DNA has vast difference like different number of chromosomes leads to infertile fish.
The problem with flowerhorns is mostly because of inbreeding and less from hybridization. Few years ago, flowerhorns used to lives more than decade and typical lifespan of pure species. Right now they are designer fish, much like most costly dog breeds.
Diseases vulnerability is complicated. Most of the issue comes from overpriced speciality foods which are really low quality rebagged foods.