New display assembly won't lay flat

I’ve had this problem twice now and cannot for the life of me figure out what’s causing it:

After display assembly replacement for the iPad Mini 4 the new display will not lay flat on the housing. it pops up at the lower right hand side as if something is pushing it up and the adhesive will not hold it. At first I thought this was the fault of a bent frame, but after much shaping and reshaping, I’ve ruled that out. the frame is immaculate. I then went through and checked the routing on all flex and antenna cables, which is also perfect. What is causing this? I can do an iPad digi repair in my sleep but this one has me stumped. Any input would be a help. Never had this issue with Mini 4 repairs before, but now seeing it on the second device in as many weeks.