Catch Starter Free – Header Image not displaying in WP 5.5 (multisite)

Hi Catch Starter Free (on Wordpress 5.5, multisite) isn't displaying the header image. I can see the link in the source code, but the image has zero height, unless I explicitly set it in the CSS. I was going to roll it out to a multisite network, but might go with Clean Blocks instead, which is working fine. I don't have time at the moment to check for plugin or theme conflicts, unfortunately, otherwise I could give you more info. Maybe it's something you can check at your end?