Hi Gabor
The underlying problem is the WFS standard is versioned (1.0.0, 1.1.0, 2.0.0...) and at various times the specification was ambiguous about coordinate axis order. Sometimes we cannot guess the correct order, so the parameter exists so you can correct the behavior in the client software.
If you can share the WFS URL here or with our support people we can show you what is going on.
Also, make sure to clear your layer cache after changing the axis order.