@tommygunz FYI for PrusaSlicer at least, two 0.4mm walls does not make a 0.8mm wall (as filament comes out with rounded edges it overlaps them). It should tell you what the wall thickness will be for a given linewidth and number of perimeters. Here's a screengrab from mine (with a 0.35mm nozzle and some non-standard line widths)
They have a knowledgebase article on it:
If you're using a 0.4mm linewidth with a 0.4mm nozzle that might give you some trouble - general advice is to have the linewidth a bit wider (e.g. 0.5mm). You use a thinner linewidth for supports when you want them to be brittle
Failing that, maybe you're printing a bit too cold so that the layers/perimeters can't bond together. Or could be you've got some mechanical backlash that means your motion isn't quite perfect (and the lines aren't overlapping enough to still bond).