Sudden Drop in GMB Ranking: Seeking Insights and Advice

Hi everyone,

I'm reaching out to this knowledgeable community for some advice and insights on a peculiar situation I've encountered with a client's Google My Business (GMB) listing and their website's local search rankings.
Here's a brief history of what's happened:

About 4 Weeks Ago: A client approached me because their Google My Business listing was no longer appearing in searches for their company.

What I Did: I verified their business, and this seemed to resolve the issue. Initially, the reviews and photos disappeared but reappeared a few hours after verification. Strangely, the problem was attributed to "inactivity," though I can't fully explain this.

New Review: Also, around four weeks ago, they received a new 5-star review, bringing their total to 11 reviews (up from 10).

Initial Ranking: Their GMB entry was ranking second for the main keyword in local searches, which was consistent even when their listing wasn't appearing for searches by name.

Ranking Drop: However, around the end of November, their local ranking plummeted from 2nd to 19th.

Website Changes: On December 6th, I almost exactly replicated the original WordPress website and migrated it to Next.js. The Lighthouse scores have been optimized, with performance improving from about 76 to 99, and other metrics at 100.

I'm perplexed by this sudden drop in local search rankings. Did I do something wrong? Could the website change have impacted their GMB listing? I'm looking for any advice, tips, or insights that could help me understand and rectify this situation.

Thank you in advance for your help!