Tonemapping is the term convert an HDR color (very bright and very dark as it happens with lighting) to an LDR color (limited by range as monitors cannot be brighter than white). Is this what you want to disable? The tonemapper curve is softly clamping off the whites and does a slight contrast enhancement.
I assume you don’t want eye adaptation. If you set min brightness and max brightness to 1 in the eye adpatation settings it will be fixed. In the editor there is a menu where this can be done as well - this is useful for editing purpose.
If you have the editor feature enabled (you can find it in the viewport menu) - you get a different behavior in game/editor.
LPV should not affect this.