Inconsistent brightness per segment on 4 digit 7 segment display

@SimonSapin wrote:

I bought "4 Digit 7 Segment Display - Blue", although it doesn’t appear to be on sale on the website anymore. The datasheet says "Model No.: USD-439AB4B-35".

I wired it to GPIO pins of a BBC micro:bit, and made a program that turns on every segment (except the dot) on each digit in turn. As far as I understand, each segment should be on the same amount of time (one fourth) and so they should all have the same brightness. But that’s not the case in practice, see the photos below (with and without a colored light filter on the display)

Could the problem be with my circuit accidentally adding resistance somewhere, or is the display malfunctioning?

both.jpg2400x1600 546 KB

Extract of my program’s source code (in Rust):

    macro_rules! pins {
        ( $($name: ident $const_: ident,)+ ) => {
            $(
                let $name = gpio::Pin::output(breadboard::$const_);
                $name.set_low();
            )+
        }
    }
    pins! {
        a DISPLAY_SEGMENT_A,
        b DISPLAY_SEGMENT_B,
        c DISPLAY_SEGMENT_C,
        d DISPLAY_SEGMENT_D,
        e DISPLAY_SEGMENT_E,
        f DISPLAY_SEGMENT_F,
        g DISPLAY_SEGMENT_G,

        d1 DISPLAY_DIGIT_1,
        d2 DISPLAY_DIGIT_2,
        d3 DISPLAY_DIGIT_3,
        d4 DISPLAY_DIGIT_4,
    }
    loop {
        for d in &[d1, d2, d3, d4] {
            d.set_high();
            busy_loop::wait_approx_ms(1);
            d.set_low();
        }
    }

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