This snippet worked for me when I just tested it:
.post_teaser_image_preview a {
max-height: unset;
}
Basically, any css rules in your custom css overwrite the rules specified elsewhere.
Edit: I just wanted to add why the max-height was set initially. This is the codeberg issue that caused there to be a limit. Basically, without setting a max-height, really tall and skinny images can take up a huge amount of vertical space.