Strange line separation in center of print  

Strange line separation in center of print

Prusa i3 MK3S

I am printing a case (PETG, 0.4 nozzle, 0.3 layer height) , which is basically a large rectangle that fills up 95% of the print bed. The initial 10 or so test prints worked fine but then I started to get some line separation in the center of the print area and it got progressively worse. It's always the same area, the other parts of the first layer look perfect.

Oddly, the second layer (and above) do not seem to have the same issue. But since the first layer is actually the front-face of the case it's a big issue.

I swapped out the print bed (smooth, steel), no difference.

I did a new XYZ calibration and selected 9 calibration points, no difference.

I changed fill angle to 90 degrees, no difference

I switched to PLA and while not as pronounced, the problem is still there.

Does anybody have any idea what could cause this?

Thanks