When a structure name is entered in the Save mode of the structure block,
The structure is unable to be loaded when switching to Load mode even after removing and replacing the structure block. The issue is solved by resaving the structure and then reloading it or leaving the world.
Steps to reproduce:
Open a structure block and save a structure
Go somewhere else and place another structure block
Write the name of the structure in save mode
Switch to load mode
The structure will not load although the 3D render on the right will adjust to the actual structure size, but the load/export button will be grayed out
I am not able to reproduce what you describe here. However, a similar issue is confirmed--see MCPE-101055.