Hi, I have set 24 products per page, and on desktop it always changed dynamicly on the first page, but after same theme settings it ok again. After a little time it happening again.
Please deactivate all the 3rd party plugins and activate only theme-required plugins on the site and then check the issue. I am sure your issue will be solved. Then Activate the 3rd party plugins one by one and check which plugin is creating the issue for you.
Otherwise, if the issue still exists then keep the 3rd party plugins deactivated and share the wp logins details so i will check and give you a possible solution.