Hi @john john ,
You could try to troubleshooting this issue by following steps:
1.Check if the corresponding managed metadata columns are added in the library and the columns have been filled with values for the items.
2.The items in the library can be crawled and searched.
3.Do a full crawl in Search Service Application and then check the results.
4.If above cannot work, please reset the search index and then do a full crawl.
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