You should evaluate your situation and do what you think is best for your birds. It’s a difficult decision.
Typically in mycoplasma infected flocks the chicks are born with the disease which then takes 10-21 days to develop. In my case, and from the sounds of it likely in yours too, they came...
It is likely that if all the birds came from the same breeder they all have it, especially if they have been raised with the sick bird and exposed to the same things it was.
Sounds like there are probably different testing requirements there.
As to recurring issues: Mycoplasma is a chronic disease. Once a bird has it, they will have it for life. Symptoms may reappear during times of stress. It will spread the disease to other birds it is in contact with. Those...
If one has it, it'st likely that they all do. You should first get testing. If they are determined to have it the responsible thing to do is either cull them or maintain a closed flock. I dealt with MS recently, here is my thread on what happened and what I did...