There could be several reasons why using the Search bar isn't working as you expected. The Search bar uses the AI-powered search which is only available on the cloud server, whereas the Filter bar search is carried out locally on your computer. So if, for example, you have syncing paused for a while, the recent changes that you made may not yet have synced to the cloud, so would not be available when using the Search bar....but would be available using the Filter bar.
Another issue may be how you use the search bar.....if you type the keyword name you will usually be given a drop down list of various items which contain the search term, such as the metadata fields. And there might also be an "untagged" entry which would represent the AI search results of images that might contain the required entity.
But you should get more accurate results by clicking in the search bar to get the drop-down list of all the search "facets" that are available, then clicking on the Keyword facet to get the list of keywords. But again, the data has to be in the cloud for the Search to work fully.