Looks like a problem with this topic, where part of a url results in an excerpt with a long unbroken line:
ref_src=twsrc^tfw|twcamp^tweetembed|twterm^1793429561969193338|twgr^|twcon^s1_&ref_url=
Discourse is not removing the url from the excerpt and there is no max-width on the column so the word-wrap: break-word; doesn’t apply.
You might be able to fix this by adding this CSS which seems to force the long line to break even without a max-width:
.topic-list .topic-excerpt span {
overflow-wrap: anywhere;
}
(My theme component also has a single line setting which would avoid this issue)