Sidebar recent posts images issue

Hi,

The images that are supposed to be round in the recent posts are not round any more. I think this has started with the release of WP 5.5. It is almost as if the image property from the block editor is leaking into the recent posts. I’ve tried browsing for the CSS code causing this using Chrome developer mode but failed miserably.

Any help would be very much appreciated!

Note, the following custom css is in use (through additional CSS);

/* zet achtergrond naar wit */
body {
    background-color: #fff;	
}
/* header achtergrond naar wit */
.header-wrapper {
    background-color: #fff;
}
/* verwijder spacing om logo */
.header {
    padding: 0px 0px 0px 0px;
}
/* verwijder spacing om logo */
.blog-logo {
	  margin: 0px 0px 0px 0px
}
/* centreert sidebar inhoud */
.sidebar {
    padding-left: 2%;
    padding-right: 2%;
/* zet sidebar kleur naar grijs */
	  background-color: #f1f1f1;
}
/* center featured images */
#posts img {
    display: block;
    margin-left: auto;
    margin-right: auto;
/* resized featured images */    
	  width: 250px !important;
    height: 125px !important;
}
/* geen categorieen op homepage */
.post-categories {
    display: none;
}
/* centreer blog titel op home */
.post-title {
	  text-align:center
}
/* centreer datum op home */
.post-meta {
	  text-align:center
}
/* Titel links binnen blog */
.single-post .post-title {
	  text-align:left
}
/* Meta links binnen blog */
.single-post .post-meta {
	  text-align:left
}
/* geen read next in post */
.single-post .related-posts { 
    display: none; 
}
/* geen featured image in post */
.single-post .post-image { 
    display: none; 
}
/* geen categorieen in post */
.single-post .post-categories {
    display: none;
}
/* gebruik ruimte van read next */
.single-post .post-inner { 
    padding-left: 0; 
	  padding-right: 0;
}
/* geen zwarte box onderaan */
.single-post .post-author {
    display: none;
}
/* verklein h1 van 2.5em naar 2em */
.single-post .post-title {
    font-size: 2em;
}

The page I need help with: [log in to see the link]