Yes, you are correct. The gap only happens when the printer starts a new layer in that corner. As for the wall thickness, I understand that as long as it's bigger than 0.8 which is the nozzle size, it should print fine. But that's not the case here.
Could it be the retraction settings? I left them at default. I would like to maintain the wall thickness setting but I am not sure what other issues could help fix such error and why would the core extrude less filament at the beginning of the each layer? One more thing for the print head temperature Its set according to Col