Previous umbilical hernia repair and muscle tightening, why are the muscles still separated? Could it happen after TT? (Photo)

It is very difficult to give you good advice based on the limited photograph and based on the limited information of what was previously done. Generally speaking, with tummy tuck surgery, the entire length of the abdominal wall musculature/fashion is exposed allowing for accurate plication of the abdominal wall fashion from the xiphoid down to the pubic bone; this complete plication was likely not performed during the limited exposure procedure you have  had done previously.  The risk of separation of abdominal wall fascia plication after tummy tuck surgery should be very small.You will do well achieving your long-term stable weight  (if you are not there already) prior to proceeding with tummy tuck surgery. You may find the attached link, dedicated to tummy tuck concerns, helpful to you as you learn more. Best wishes.

It is hard to say what is currently going on without an examination and not knowing exactly what procedure was performed initially.  In my experience, my patients do not have recurrence of the diastasis after a full abdominoplasty.  Good Luck, Gaurav Bharti