Steps to reproduice the issue :

Settings
Set the search engine as follow :

• Choose a content on which to do the search
• In "Search by tag", choose a tag Category
• Choose number of result by page so as to obtain a pagination

Testing

• Let keyword field empty
• Set a tag value in the dropdown
• Launch the search => The results are displayed with pagination
• Click on "Next page" or on other page value => The tag filter is not applied anymore and the results displayed are now all the ones corresponding to the content type set.