Have there been any recent changes in the Google Maps results algorithm? One of my client's Top 3 results have changed dramatically in the last couple of months, and I notice the placements seem to change on a daily basis. Can no longer maintain a consistent ranking.
There have been some general changes in Google algorithms in November and I think this had a huge impact on different sites and different areas of doing SEO. So, I guess your client faced a drop in ranking because of this, that would be my guess at least. If he didn't change anything on the site recently, the problem shouldn't come from there, but a good idea would be to also check competitors and their behavior, as if they changed something on their site, they might jump on rankings and pull you down.
This issue is unrelated to Organic Search. The organic search results are relatively unchanged. I am talking specifically about the Local Results.
Start embedding met data into the images you use on the GMB profile - GPs coordinates of the registered office is a big one.
Tons of tools can do it, easiest is probably this one: make sure it matches exactly what Google Maps say the coordinates of the registered office is. (even if its a little off from the reality, go with what they say) https://www.batchphoto.com/blog/how-to-add-gps-coordinates-to-photos/