Yes, but you may have selected a different poster manually while the library was still using the old agent. And after the conversion it has retained the number of the poster, even though now this poster number has a totally different subject. Because all manual changes result in a “lock” of the changed meta datum.
If you inspect the plex media info XML of the show, you may see this lock as <Field locked="1" name="thumb"/>
Depending on whether this is a single instance or if there are dozens of shows affected, you can either select a more suitable poster manually,
or you can release the lock on all posters of the library at once: Reset all manual poster selections in a library at once or unlock other metadata fields
