I tried to make a beacon powered by plasma, so I added a filter for "fusion-plasma"
but when I tested it, I was also able to power it with steam

Can you send me the prototype you used?

sure!

{
        name = "fusion-beacon",
        type = "beacon",
        icon = "__base__/graphics/icons/beacon.png",
        flags = {"placeable-player", "player-creation"},
        minable = {mining_time = 0.5, result = "fusion-beacon"},
        collision_box = {{-1.2, -1.2}, {1.2, 1.2}},
        selection_box = {{-1.5, -1.5}, {1.5, 1.5}},
        allowed_effects = {"consumption", "speed", "pollution"},
        graphics_set = require("__base__.prototypes.entity.beacon-animations"),
        energy_usage = "25MW",
        energy_source = {
            type = "fluid",
            fluid_box = {
                volume = 25,
                filter = "fusion-plasma",
                pipe_connections = {
                    {
                        direction = 0,
                        position = {0, -1},
                        flow_direction = "input-output"
                    },
                    {
                        direction = 4,
                        position = {1, 0},
                        flow_direction = "input-output"
                    },
                    {
                        direction = 8,
                        position = {0, 1},
                        flow_direction = "input-output"
                    },
                    {
                        direction = 12,
                        position = {-1, 0},
                        flow_direction = "input-output"
                    }
                }
            }
        },
        radius_visualisation_picture = {
            filename = "__base__/graphics/entity/beacon/beacon-radius-visualization.png",
            priority = "extra-high-no-scale",
            width = 10,
            height = 10
        },
        supply_area_distance = 3,
        distribution_effectivity = 1.5,
        module_slots = 2,
    }

I... don't know why it's doing this. The energy source should be respecting the fluidbox filter. I believe that its something to do with missing energy_source properties? I don't know why, cause the steam example does work... Might need to just try messing around with things until it works. That or reporting it to the devs.

It's also missing the destroy_non_fuel_fluid value, but that shouldn't matter... I don't know why it's doing that.

Ah, figured it out. Fluidbox filters are not respected when fluidbox.production_type is not set (it should be input). So if you set that property, then only fusion plasma will be permitted. On being able to power it with non-plasma fluids, the fluidbox filter isn't technically a power source filter, it just affects the fluidbox. I feel like that should probably be a feature request... gonna go make that now.