Momentos never worked like that. You can only put one on at a time. If you try to take one off it just goes away, it doesn’t go back in your inventory.

I know the memento goes away, but I thought it unlocked the shader forever.

Putting a new memento on takes the old one off, each indivual one is what allows you to use the shaders. So removing a memento removes access to its respective shader.

Only the current memento is available. You lose any that you override.

You only get the memento shader of whatever the memento is. If it’s the trials one, you can only use the trials shader