My theory why it is happening is from one picture angle anything built into that walls as they can move over time so might be the case with in wall units (I am from the northern climate where my walls can expand/shrink shift/move in the summer/wintertime to my frustration as new homeowner learning how older houses works)
So its a bull nose style tank If it curves into a D ? Thats kind out of my experience range as im used to rectangular tanks
I am not confident you can do this tank safely even seeing cracks on the topside With bull nose just every WC you are doing is causing it to flex in and out- but its just on borrowed time we dont have the picture of the tank as a whole unit anyways
Like
Lepisosteus
said the seam is already separated where is white thats a red flag do you have the company info who custom made this?