Hi Pavel,
I have the latest WP 5.4.1 installation, WC 4.1.1, Latest ATUM Plugin, running on 7.3.18 PHP Version, MSQL version 5.6.47.
I have disabled all plug-ins apart from the WC and ATUM plugin. I tried it, but no luck.
I made a screenshot for you to see what I mean. The product A5003 was out of stock, as the above on 0. I put on 44 and saved it. But after saving it the red circle isn’t changing to green and the product doesn’t go from “Out of stock” to “In Stock”. Don’t know why…
https://www.atlas-repropaperwork.com/screenshot.jpg