When creating a recurring invoice, the "add an item" field appears when the mouse is hovered over any of the white area of the "editor-content" class div section. When you click on "select a product" so the dropdown appears, and then try to select a product that is at the bottom of the list, this now leaves your mouse off the white editor area so the "add an item" box disappears and the product is not added. Either the editor content needs to be taller, to leave room for the dropdown to still be above the white area, or the dropdown scroll area needs to be shorter, showing maybe only 4 products in the view and then we scroll from there.

See Loom recreation here: https://www.loom.com/share/16468a3d1c804f778dc0b01140cba8ab