In my own experience with Pages v8.1 on macOS 10.14.6 and using an Apple Magic Mouse II, the insertion beam remains visible as long as I am typing content, and have not stopped and inadvertently bumped the mouse or keyboard focus from Pages.
When you insert any document object, the current focus shifts to this new foreground object, and the insertion beam vanishes from your text. As soon as you click back on the document, and the inserted object is relegated to the background, then the insertion beam returns where it was last visible.
If you are using a Wacom input device, there is a long storied history of insertion beam aberrations when using just about any version of Pages.