OK, great! So it appears there are some replicable problems here which is good. That will help engineers hopefully with finding the source of the problem!
I am moving this to Bug and slapping staff-experience on it so the staff experience team can take a look when they get a chance.
“Korda” tag on carptalk.org works correctly - the onebox description is the same as the description you see when you visit the page. https://forum.carptalk.org/tag/Korda
“Horcott-lakes” does not work correctly - the onebox description is the default, as though there is no description specified. But when you visit the page you see the description is specified. On that page though the meta description does not correctly show the description but rather <meta name="description" content="Topics tagged horcott-lakes"> . https://forum.carptalk.org/tag/horcott-lakes
Here on meta none of the tag oneboxes we’ve tried is working at all here in this topic - just the URL is displaying. e.g. https://meta.discourse.org/tag/release-notes has a description that you can see on the page.
The expected behavior is that in all these cases the onebox will show and also contain the description specified for the tag.