Hello Saljain,
I assume its due to having more than 1000 distinct values. Normally QuickSight will show all values in a list, but if there are more than 1,000 distinct values it will instead show a search box. If you add this filter to a dashboard or pin it, it will also give users a search box … this may be similar to old post - please refer - Quicksight Filter not working because of too many values
I would also recommend filing a case with AWS Support where we can dive into the details so that we can help you further and recommend best approaches based on your scenario. Here are the steps to open a support case. If your company has someone who manages your AWS account, you might not have direct access to AWS Support and will need to raise an internal ticket to your IT team or whomever manages your AWS account. They should be able to open an AWS Support case on your behalf.
Hope this helps!
Cheers,
Deep