This is possibly a graphical bug but I am not sure.
to recreate this.
1-make sure score zoom % is set at 50%
2-Select the first page in engrave mode
3-click on the insert page
4-Dorico adds a Blank page but somehow hides my other page
5-If I zoom out and Zoom in or go to Write mode and come back to engrave mode my missing page comes back.
***This only happens sometimes when zoom % is 50% but never on other zoom %.