large seam line in print near bed on several prints

[hardcak]

I have printed the same part trice now in similar orientations and supporting and the same defect is appearing. the back of this part is suppose to be flat, but 20mm or so up parallel to the plate appears a relatively large dip across the part, followed by some sloping/sagging. Im no expert in supporting resin parts as I have only purchased the printer 2 weeks ago (EM2 pro)..but I think its supported well.!? Could this be a hardware fault as I think its happen on another part when I first started printing. strange as the small part next to it is perfect, and other prints are fine. A link to the CHITO project file and pictures below. Any pointers would be appreciated.

https://we.tl/t-gosQYti3bd