I think those conflict as you are defining a fixed height. If you want to keep the image ratio, use width: auto;
Min or max-content is better used on a parent container than an element itself. Although you can do that.
I haven’t checked with repository manager where the CSS file is for it, but I wouldn’t edit it there, as an update will likely restore what you edit. Create an override, but then there is probably a practical reason, the thumbnail is fixed width and height, I haven’t used it for a while and can’t recall the interface. But I’m sure @jannis can clarify.
Hey Pete because I changed them to what they ought to be and I can now see the thumbnail images. I could be wrong but I think the export for RM is keeping its settings.
You ask why is because both of those settings were 0 when they should have been 1