Hi grandtam & leiferikg,
I have tested this on our test site with only WooCommerce, WPCstore theme and WPC plugins installed. It is working properly.
The quantity of the main product is updated when choosing the quantity on the sticky atc bar and the quantity on the Fly Cart popup as well as the cart page is the same.
Please check the screenshots: https://prnt.sc/msQ9nDpx1KLB & https://prnt.sc/S8PVE_wADawf.
If you are using any external quantity controlling or dynamic pricing/discount plugins, they can prevent our plugin from functioning properly.
You can also try to keep only WooCommerce and our plugin activated then re-activate other plugins on your site to find which is interfering with our plugin.
Best regards.