The docs i was referring to was:
How much this progress bar is filled. It is a value in range [0, 1].
Can only be used if this is progressbar
https://lua-api.factorio.com/latest/LuaGuiElement.html#LuaGuiElement.value

And:
The value of this slider element.

Can only be used if this is slider ...

When adding a gui element of type slider the property value must be set in order to have a starting value for the slider, slider_value doesnt work. But later value cannot be accessed at all. You have to use slider_value which actually has the value previously set using the property value . This is ...