It looks like fertilizer burn to me. Are you in a colder climate zone? And did you fertilize before winter came? All my Myrtaceae that overwintered outside look like that. They get the spots when the weather gets really cold.
I think what happens is that the slow release fertilizer keeps releasing fertilizer in the ground with every rainfall in winter, and the roots keep taking up the nutrients, but the top part of the plant has stopped growing or is dormant, so there is an inevitable build up of nutrients inside the plant. And the salt build up shows up as deep red lesions on the leaves.
This may be a particular problem to evergreens that keep their leaves in winter, but slow down or halt growth.
Now that the temperatures pick up and the plants are actively growing again here, all new leaves look healthy, so I don't think it is indicative of permanent damage, or a real problem. But the lesions may make it easier for fungi to establish on your plants.