A Blunder Committed on Search Console

One of my SEO guys committed an unforgivable blunder.

He forgot to put the sitemap in the google search console at the start of the sprint 3 months back and reported that it was done. We are still ranking for a few keywords that have high KD on the top SERP page as pages are indexed. Theoretically, if we submit a site map it should boost our SEO but no one has ever seen this type of thing.

Will our SERP rankings get affected if we submit the sitemap now or will that have no effect. We have worked very hard to get the rankings and do not want to see them plummet because of this.

Any Opinions or has anyone gone through it?