agvtool seems to be the culprit. If I comment out its call in my pre-action script in the "Build" step in my scheme, the problem no longer occurs for me.

I have a similar situation, but for me it's "only" around a 25% drop since the beginning of March (I've never had sales in China). It affects all my Mac apps, the decline is fairly evenly distributed. I suspect that it is related to a changed search mechanism in the Mac App Store on Silicon Macs, where (now -- starting with macOS 14.4?) the search results for iPhone and iPad apps are often presented first, depending on the search term entered. This means that the user then has to explicitly switch to the "Mac Apps" tab first in order to see the Mac apps found at all.