Twenty seventeen – header image on mobile devices is scaled wrong

Dear community, I have a „scaling“-problem with the header image/meta slider gallery in twenty-seventeen…
I have already posted this topic in another forum, where it was closed and the admin suggested to place this here…
I am currently building a website (see link) with a Meta Slider-gallery in the header, I implemented this via (header-image.php):

<div class="custom-header">
		<div class="custom-header-media">
			<?php echo do_shortcode('[metaslider id="105"]'); ?>
		</div>
	<?php get_template_part( 'template-parts/header/site', 'branding' ); ?>
</div>

The images in the slideshow have a size of 1920x824px and on my 1920px-width-screen, everything looks fine, but, on devices with a smaller width, the images of the slideshow become horizontally „compressed“ – and this also happens, when simpy defining a background-image in twenty-seventeen via „design“>“customizer“>“header media“>“header image“… when lowering the screen-width, the image is being scaled unproportionally… is there any known solution for this???
Thank you very much!

This seems only to happen in FF (v42.0), in Opera and Chrome, it seems to work….