We checked on one of the packages pages at https://www.nepalpyramids.com/category/package/nepal/nepal-helicopter-day-tours/ and the title showing in the source code is:
<title>Nepal Helicopter Day Tours Packages - Cost and Detail Itinerary | Nepal Pyramids</title>
Can you please confirm whether that title is correct?
As for how the title displays in the search results, Google typically uses the title from the metadata for indexing but this is not always the case. Google will sometimes modify the title of the URL as it sees fit, as well as sometimes adding the brand or sitename to the end of your title. This is not due to the plugin not functioning as expected, but rather is due to Google modifying the Title it indexes as it’s algorithm has concluded that there is a better version to index.
We also have additional information on this topic available at the following links: https://yoast.com/google-page-title/ and https://kb.yoast.com/kb/google-shows-different-titles-for-my-site/. Please refer to these articles as the include recommendations for crafting a page title which will help increase the likelihood that Google will use the title you specified instead of modifying your page’s title as it sees fit.