Hi there,
Sometimes, parts of the plugins like generated stylesheets and/or things from database keep a static entry w/ literal text strings of what it saves. Some of these strings are likely URLs and they weren’t converted do the destination site’s URL when you migrated.
There are tools for this. The most common is this one – https://wordpress.org/plugins/better-search-replace/
What this does is, it lets you search for entries w/ the old domain and replace ALL instances with the new domain.