Google bug, how to fix a search bug??

Hi

I ranked for a very long time 1st place for a competitive search phrase on mobile and desktop

All of a sudden about two weeks ago, we no longer rank 1st on Desktop but STILL rank 1st on Mobile

It seems impossible given Google officially said they phased our desktop search results and mobile were the authority

yet here we are mid way through 2021 where there are two completely different search results for the same phrase. What on earth am I supposed to do to get back my lost 1st place listing from a desktop search?

Weirdly, Google webmaster tools says the URL is not in the index… yet it is when I search it direct. Then when looking at the URL via one of the other options in webmaster tools it says everything is fine and no issues.
I requested a reindex, made no difference.

Two weeks on, still #1 when searching from mobile
Still no where when searching from desktop

the page itself has nothing special about it that’s mobile focused, literally just a single mobile responsive meta tag and that’s it. Same content for rest of page.

Any ideas????
SEMrush