How to structure a sitemap by region (regional site versions)?

Question

Create a sitemap

Good time!
Please help to solve the issue of building a sitamap for an online store with regional versions of the site.
At the end of July, we launched regions (27 cities) and added these pages to the sitemap, dividing it into each region, each sitemap of the region has 10K URLs. Before that, our sitemap was simply divided into 10K URLs, there were 13 files in total.
After adding regions to the sitemap, our traffic (Google and Yandex) dropped sharply every week, minus 10%.
We think the reason is the wrong division of the sitemap.
Please let me know if anyone has experience and how to do it right. Our site talapai.kz (talapai.kz/sitemap.xml)
will be glad to any suggestion.