I still can't overcome the issue. Almost all the posts have this issue where I use Thrive Architect. I can assure, the problem is not with Thrive Architect since such issues never happened with Yoast SEO. Please come up with a permanent solution.
I tried removing the content and adding it again, clearing cache, but still, Rank Math can't find the focus keyword added. I tried other methods, adding content using WordPress editor block in Thrive Architect, but still, the issue continues.