On an older Formlabs printer I’d theorize that’s simply too big a footprint and suggest printing on supports, at an angle.
I don’t have as much experience yet with large, direct-on-base prints on the Form 3. I’d hoped the gentler peel mechanism would accommodate big footprints, and to that end I was able to do a huge direct-on-base print on the 3L in Rigid 4000.
Angling it would probably still solve the problem.
The impression I got from those photos and your description is that the first few layers stuck to the platform, before remaining ones started to peel back from the ones below (in one corner). If so, it’s probably not a build platform flatness issue. If nothing adhered at all in some portions of the print then it might be, and sanding your platform could potentially help. But I’d reach out to Support first to see what they think (and ask them to check how your laser power looks to eliminate the possibility of optical window contamination). Here’s some more info on the failure modes you might be seeing: