I want to invert the mouse direction but the "Invert Mouse Y" option doesn't do anything.

BlueMap: 5.7
Minecraft: 1.21.1
Loader: Fabric
OS: Windows 10
Browser: Vivaldi
Server runs on my pc

It's not quite clear, i know, but this setting works only for the right-click-drag control in first-person view, or when you are in mouse-capture mode (single click into the screen when in first-person-mode to enable mouse-capture).
But left-click-drag (as you are doing here) is not intended to be affected by the invert mouse y setting :)
So this actually works as intended.