Please go to My Site → Customize → CSS (or Additional CSS), then add all of the following code to the bottom of that window.
/* This CSS fixes the theme issue with centered image blocks.
* 10230156-hc (jw)
*/
.post-content .aligncenter {
text-align: center;
}
.wp-block-image .aligncenter>figcaption {
display: block;
}
Tip: keep the /*
comment */
in the code to later know what this code does.