Google is still not completed with mobile first indexing even till the end of May 2021.
Google’s John Mueller, said this on Twitter. It seems the team is still working on few issues with some sites. That’s why its bit behind. John Mueller cleared this when a question was asked why Search Console is still showing the Desktop indexing & not the mobile ones.
Here are the Tweet Chatter
Well, that was the plan :-). It looks like the team is still working through some issues with some sites, so it's all a bit behind. Did you get any issues sent via Search Console in the past?
— ? John ? (@JohnMu) May 25, 2021
It seems its taking Google too much time to transfer everything from Desktop to Mobile Indexing. Google initiated this process near about four & a half year ago.
Well, No one can clarify that when it will fully complete? But assumptions will be made that hopefully it will be complete by 2022.