I'd write: "any further failure to attend an appointment" and 'deregister'. I don't think it matters whether you put 'being' or 'becoming'. "Being deregistered" implies that someone will deregister you. "Becoming deregistered" implies that the patient's status will change. One implies the other in this context.
'Pt' as an abbreviation for 'patient' does not require either letter to be a capital. But PT is wrong, as is pT. PT is acceptable.
Just thought that you might like to know that here in the United States (or at least in my part of the country), it is more natural to refer to a failure to keep an appointment. (Until I read your post, I had never heard of anyone "attending" an appointment. Thank you for teaching me that expression.)